1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
------------------------------------------------------------------------------
--- Library for constructing static and dynamic HTML pages.
--- [This paper](https://www.informatik.uni-kiel.de/~mh/papers/PADL01.html)
--- contains a description of the basic ideas behind an old version
--- of this library.
---
--- An application written with this library can be transformed into
--- a cgi script by the command
---
---     > cypm exec curry2cgi -m mainPage -o /home/joe/cgi-bin/prog.cgi Prog
---
--- where `Prog` is the name of the Curry program with
--- the cgi script, `/home/joe/cgi-bin/prog.cgi` is
--- the desired location of the
--- compiled cgi script, and `mainPage` is the Curry expression
--- (of type `IO HtmlPage`) computing the HTML page (where `cypm`
--- is the command calling the Curry Package Manager).
---
--- @author Michael Hanus (with extensions by Bernd Brassel and Marco Comini)
--- @version October 2020
------------------------------------------------------------------------------

{-# OPTIONS_CYMAKE -Wno-incomplete-patterns #-}

module HTML.Base
 ( HTML, htmlText, htmlStruct, hStruct, updAttrs, toBaseHtml,
   HtmlExp(..), BaseHtml(..), toHtmlExp, fromHtmlExp, textOf,
   HtmlPage(..), PageParam(..),
   FormReader, fromFormReader, toFormReader,
   HtmlFormDef, simpleFormDef, simpleFormDefWithID, formDef, formDefWithID,
   formDefId, setFormDefId, formDefRead, formDefView,
   CookieParam(..),
   HtmlRef, idOfHtmlRef, instHtmlRefs, HtmlEnv, HtmlHandler,
   defaultEncoding,
   answerText, answerEncText,
   getCookies,
   page, headerPage,
   pageEnc, pageCookie, pageCSS, pageMetaInfo,
   pageLinkInfo, pageBodyAttr, addPageParam, addCookies, addHttpHeader,
   htxt, htxts, hempty, nbsp, h1, h2, h3, h4, h5, h6,
   par, section, header, footer, emphasize, strong, bold, italic, nav, code,
   center, blink, teletype, pre, verbatim, address, href, anchor,
   ulist, ulistWithClass, ulistWithItemClass,
   olist, olistWithClass, olistWithItemClass,
   litem, dlist,
   table, tableWithClass, headedTable,
   hrule, breakline, image,
   styleSheet, style, textstyle, blockstyle, inline, block, hiddenField,
   redirectPage, expires,
   formElem,
   button, resetButton, imageButton, coordinates,
   textField, password, textArea, checkBox, checkedBox,
   radioMain, radioMainOff, radioOther,
   selection, selectionInitial, multipleSelection,
   htmlQuote, htmlIsoUmlauts, addAttr, addAttrs, addClass,
   showBaseHtmls, showBaseHtml, showHtmlPage,
   htmlPrelude, htmlTagAttrs,
   getUrlParameter, urlencoded2string, string2urlencoded,
   formatCookie
 ) where

import Data.Char          ( isAlphaNum, isSpace )
import Numeric            ( readNat, readHex )
import System.Environment ( getEnv )
import Data.Time          ( CalendarTime(..), ClockTime, toTimeString, toUTCTime )

infixl 0 `addAttr`
infixl 0 `addAttrs`
infixl 0 `addClass`
infixl 0 `addPageParam`

------------------------------------------------------------------------------
--- The default encoding used in generated HTML documents.
defaultEncoding :: String
defaultEncoding = "utf-8" --"iso-8859-1"

------------------------------------------------------------------------------
-- Basic types for (static) HTML documents.

--- The attributes for HTML structures consists of a list of
--- name/value pairs.
type Attrs = [(String,String)]

--- The data type to represent static HTML expressions.
--- @cons BaseText s         - a text string without any further structure
--- @cons BaseStruct t as hs - a structure with a tag, attributes, and
---                             HTML expressions inside the structure
--- @cons BaseAction act     - an action that computes a general HTML expression
---                            which will be inserted when the HTML document
---                            is shown (used to implement form expressions)
data BaseHtml =
    BaseText   String
  | BaseStruct String Attrs [BaseHtml]
  | BaseAction (IO HtmlExp)

--- Updates the attributes in a basic HTML expression.
updBaseAttrs :: (Attrs -> Attrs) -> BaseHtml -> BaseHtml
updBaseAttrs _ (BaseText s)                 = BaseText s
updBaseAttrs f (BaseStruct tag attrs hexps) = BaseStruct tag (f attrs) hexps
updBaseAttrs _ (BaseAction act)             = BaseAction act

--- A type is an instance of class `HTML` if it has operations to construct
--- HTML documents, i.e., constructors for basic text strings and
--- structures with tags and attributes, update the attributes in
--- structures, and extracting the basic HTML structure, i.e.,
--- transforming a document into a basic HTML document.
class HTML a where
  htmlText   :: String -> a
  htmlStruct :: String -> Attrs -> [a] -> a
  updAttrs   :: (Attrs -> Attrs) -> a -> a
  toBaseHtml :: a -> BaseHtml

--- An HTML structure with a given tag and no attributes.
hStruct :: HTML h => String -> [h] -> h
hStruct tag = htmlStruct tag []

--- The type of basic HTML expressions is an instance of class `HTML`.
instance HTML BaseHtml where
  htmlText   = BaseText
  htmlStruct = BaseStruct
  updAttrs   = updBaseAttrs
  toBaseHtml = id

------------------------------------------------------------------------------
-- CGI references and environments.

--- The (abstract) data type for representing references to input elements
--- in HTML forms.
data HtmlRef = HtmlRef String

--- Internal identifier of a HtmlRef (intended only for internal use in other
--- libraries!).
idOfHtmlRef :: HtmlRef -> String
idOfHtmlRef (HtmlRef i) = i

--- The type for representing cgi environments, i.e., mappings
--- from cgi references to the corresponding values of the input elements.
type HtmlEnv = HtmlRef -> String

--- The type of event handlers occurring in HTML forms.
type HtmlHandler = HtmlEnv -> IO HtmlPage

------------------------------------------------------------------------------
--- The data type for representing HTML expressions with input elements,
--- i.e., all elements which might occur inside a form.
--- @cons HtmlText s           - a text string without any further structure
--- @cons HtmlStruct t as hs   - a structure with a tag, attributes, and
---                              HTML expressions inside the structure
--- @cons HtmlInput ref h      - an input element (described by the second
---                              argument) with a cgi reference
--- @cons HtmlEvent h ref hdlr - an input element (first arg) identified
---                              by a cgi reference with an associated
---                              event handler (typically, a submit button)
--- @cons HtmlAction act       - an action that computes an HTML expression
---                              which will be inserted when the HTML document
---                              is shown (used to implement form expressions)
data HtmlExp =
    HtmlText   String
  | HtmlStruct String Attrs [HtmlExp]
  | HtmlAction (IO HtmlExp)
  | HtmlInput  HtmlRef HtmlExp
  | HtmlEvent  HtmlRef HtmlHandler HtmlExp

--- Updates the attributes in an HTML expression.
updHtmlAttrs :: (Attrs -> Attrs) -> HtmlExp -> HtmlExp
updHtmlAttrs _ (HtmlText s)                 = HtmlText s
updHtmlAttrs f (HtmlStruct tag attrs hexps) = HtmlStruct tag (f attrs) hexps
updHtmlAttrs f (HtmlEvent ref handler he)   =
  HtmlEvent ref handler (updHtmlAttrs f he)
updHtmlAttrs f (HtmlInput ref he)           = HtmlInput ref (updHtmlAttrs f he)
updHtmlAttrs _ (HtmlAction act)             = HtmlAction act

--- Transforms a static into a dynamic HTML document.
toHtmlExp :: BaseHtml -> HtmlExp
toHtmlExp (BaseText s)         = HtmlText s
toHtmlExp (BaseStruct t ps hs) = HtmlStruct t ps (map toHtmlExp hs)
toHtmlExp (BaseAction a)       = HtmlAction a

--- Transforms a dynamic HTML into a static one by dropping references
--- and event handlers.
fromHtmlExp :: HtmlExp -> BaseHtml
fromHtmlExp (HtmlText   s)       = BaseText s
fromHtmlExp (HtmlStruct t ps hs) = BaseStruct t ps (map fromHtmlExp hs)
fromHtmlExp (HtmlAction a)       = BaseAction a
fromHtmlExp (HtmlEvent  _ _ hs)  = fromHtmlExp hs
fromHtmlExp (HtmlInput  _ hs)    = fromHtmlExp hs

--- The type of HTML expressions is an instance of class `HTML`.
instance HTML HtmlExp where
  htmlText   = HtmlText
  htmlStruct = HtmlStruct
  updAttrs   = updHtmlAttrs
  toBaseHtml = fromHtmlExp

------------------------------------------------------------------------------
--- Extracts the textual contents of a list of HTML expressions.
--- For instance,
---
---      textOf [BaseText "xy", BaseStruct "a" [] [BaseText "bc"]] == "xy bc"
---
textOf :: [BaseHtml] -> String
textOf = unwords . filter (not . null) . map textOfBaseHtml
 where
  textOfBaseHtml (BaseText s)          = s
  textOfBaseHtml (BaseStruct _ _ hs)   = textOf hs
  textOfBaseHtml (BaseAction _)        = ""

------------------------------------------------------------------------------
-- HTML forms.

--- The type `FormReader` is a monad with operations to read data
--- to invoke an HTML form.
--- It is assumed that a `FormReader` action reads only data and does not
--- change the environment, since the action is applied twice
--- when executing a form.
--- A typical action of this kind is `HTML.Session.getSessionData`.
--- 
--- The `FormReader` type encapsulates IO actions in order to enforce
--- the correct use of forms.
data FormReader a = FormReader (IO a)

--- Transforms a `FormReader` action into a standard IO action.
fromFormReader :: FormReader a -> IO a
fromFormReader (FormReader a) = a

--- Transforms an IO action into a `FormReader` action.
--- This operation should be used with care since it must be
--- ensured that the action only reads data and does not
--- change the environment, since the action is applied twice
--- when executing a form.
toFormReader :: IO a -> FormReader a
toFormReader = FormReader

instance Functor FormReader where
  fmap f (FormReader x) = FormReader (fmap f x)

instance Applicative FormReader where
  pure x = FormReader (return x)
  f <*> v = f >>= \f' -> fmap f' v

instance Monad FormReader where
  return  = pure
  a >>= f = FormReader (fromFormReader a >>= \x -> fromFormReader (f x))

--- The data type for representing HTML forms embedded into HTML pages.
---
--- A form definition consists of a unique identifier of form (usually,
--- the qualified name of the operation defining the form),
--- a `FormReader` action and a mapping from data
--- into an HTML expression (which usually contains event handlers
--- to produce the form answers).
data HtmlFormDef a = HtmlFormDef String (FormReader a) (a -> [HtmlExp])

--- A definition of a simple form which does not require session data.
---
--- The unique identifier required for the implementation of forms
--- is added by the `curry2cgi` translator.
simpleFormDef :: [HtmlExp] -> HtmlFormDef ()
simpleFormDef hexps = HtmlFormDef "" (return ()) (const hexps)

--- A definition of a simple form, which does not require session data,
--- with a unique identifier (usually, the qualified name of the
--- operation defining the form).
simpleFormDefWithID :: String -> [HtmlExp] -> HtmlFormDef ()
simpleFormDefWithID fid hexps = HtmlFormDef fid (return ()) (const hexps)

--- A definition of a form which consists of a `FormReader` action
--- and a mapping from data into an HTML expression
--- (which usually contains event handlers to produce the form answers).
--- It is assumed that the `FormReader` action reads only data and does not
--- change it, since it is applied twice when executing a form.
---
--- The unique identifier required for the implementation of forms
--- is added by the `curry2cgi` translator.
formDef :: FormReader a -> (a -> [HtmlExp]) -> HtmlFormDef a
formDef = HtmlFormDef ""

--- A definition of a form with a unique identifier (usually,
--- the qualified name of the operation defining the form).
--- A form contains a `FormReader` action and a mapping from data
--- into an HTML expression (which usually contains event handlers
--- to produce the form answers).
--- It is assumed that the `FormReader` action reads only data and does not
--- change it, since it is applied twice when executing a form.
formDefWithID :: String -> FormReader a -> (a -> [HtmlExp]) -> HtmlFormDef a
formDefWithID = HtmlFormDef

--- Returns the identifier of a form definition.
formDefId :: HtmlFormDef a -> String
formDefId (HtmlFormDef s _ _) = s

--- Sets the identifier of a form definition.
--- Only intended for internal use in the `curry2cgi` translator.
setFormDefId :: String -> HtmlFormDef a -> HtmlFormDef a
setFormDefId fid (HtmlFormDef _ readact formgen) =
  HtmlFormDef fid readact formgen

--- Returns the `FormReader` action of a form definition.
formDefRead :: HtmlFormDef a -> IO a
formDefRead (HtmlFormDef _ ra _) = fromFormReader ra

--- Returns the view operation of a form definition.
formDefView :: HtmlFormDef a -> (a -> [HtmlExp])
formDefView (HtmlFormDef _ _ v) = v


-- Auxiliary operations for executing forms.

--- Computes the initial form of a form definition.
genInitForm :: HtmlFormDef a -> IO [HtmlExp]
genInitForm (HtmlFormDef _ readact formgen) =
  fromFormReader readact >>= return . formgen

--- Instantiates all HtmlRefs with a unique tag in HTML expressions.
--- Only internally used.
--- Parameters: HTML expressions, number for cgi-refs
--- Result: translated HTML expressions, new number for cgi-refs
instHtmlRefs :: [HtmlExp] -> Int -> ([HtmlExp],Int)
instHtmlRefs [] i = ([],i)
instHtmlRefs (HtmlText s : hexps) i =
  case instHtmlRefs hexps i of
    (nhexps,j) -> (HtmlText s : nhexps, j)
instHtmlRefs (HtmlStruct tag attrs hexps1 : hexps2) i =
  case instHtmlRefs hexps1 i of
    (nhexps1,j) -> case instHtmlRefs hexps2 j of
                     (nhexps2,k) -> (HtmlStruct tag attrs nhexps1 : nhexps2, k)
instHtmlRefs (HtmlEvent cgiref handler (HtmlStruct tag attrs hes) : hexps) i
  | idOfHtmlRef cgiref =:= ("FIELD_" ++ show i)
  = case instHtmlRefs hexps (i+1) of
      (nhexps,j) ->
         (HtmlEvent cgiref handler (HtmlStruct tag attrs hes) : nhexps, j)
instHtmlRefs (HtmlInput cgiref hexp : hexps) i
  | idOfHtmlRef cgiref =:= ("FIELD_" ++ show i)
  = case instHtmlRefs [hexp] (i+1) of
      ([nhexp],j) -> case instHtmlRefs hexps j of
                       (nhexps,k) -> (nhexp : nhexps, k)
instHtmlRefs (HtmlAction _ : _) _ =
  error "HTML.Base.instHtmlRefs: HtmlAction occurred"

------------------------------------------------------------------------------
--- The data type for representing HTML pages. Since the HTML document
--- shown in this page is a base HTML expression, it is ensured that
--- input elements and event handlers occur only in embedded forms.
--- @cons HtmlPage t ps hs - an HTML page with title t, optional parameters
---                          (e.g., cookies) ps, and contents hs
--- @cons HtmlAnswer t c - an answer in an arbitrary format where t
---         is the content type (e.g., "text/plain") and c is the contents
data HtmlPage = HtmlPage String [PageParam] [BaseHtml]
              | HtmlAnswer String String

--- The possible parameters of an HTML page.
--- The parameters of a cookie (`PageCookie`) are its name and value and
--- optional parameters (expiration date, domain, path (e.g., the path "/"
--- makes the cookie valid for all documents on the server), security) which
--- are collected in a list.
--- @cons PageEnc - the encoding scheme of this page
--- @cons PageCookie name value params - a cookie to be sent to the
---                                      client's browser
--- @cons PageCSS s - a URL for a CSS file for this page
--- @cons HttpHeader key value - additional HTTP header included in this page
--- @cons PageJScript s - a URL for a Javascript file for this page
--- @cons PageMeta as - meta information (in form of attributes) for this page
--- @cons PageLink as - link information (in form of attributes) for this page
--- @cons PageHeadInclude he - HTML expression to be included in page header
--- @cons PageBodyAttr attr - optional attribute for the body element of the
---                           page (more than one occurrence is allowed)
data PageParam = PageEnc         String
               | PageCookie      String String [CookieParam]
               | PageCSS         String
               | HttpHeader      String String
               | PageJScript     String
               | PageMeta        [(String,String)]
               | PageLink        [(String,String)]
               | PageHeadInclude BaseHtml
               | PageBodyAttr    (String,String)

--- An encoding scheme for a HTML page.
pageEnc :: String -> PageParam
pageEnc = PageEnc

--- A cookie to be sent to the client's browser when a HTML page is
--- requested.
pageCookie :: (String,String) -> PageParam
pageCookie (n,v) = PageCookie n v []

--- A URL for a CSS file for a HTML page.
pageCSS :: String -> PageParam
pageCSS = PageCSS

--- A header to be sent to the client's browser when a HTML page is
--- requested.
httpHeader :: String -> String -> PageParam
httpHeader = HttpHeader

--- Meta information for a HTML page. The argument is a list of
--- attributes included in the `meta`-tag in the header for this page.
pageMetaInfo :: [(String,String)] -> PageParam
pageMetaInfo = PageMeta

--- Link information for a HTML page. The argument is a list of
--- attributes included in the `link`-tag in the header for this page.
pageLinkInfo :: [(String,String)] -> PageParam
pageLinkInfo = PageLink

--- Optional attribute for the body element of the web page.
--- More than one occurrence is allowed, i.e., all such attributes are
--- collected.
pageBodyAttr :: (String,String) -> PageParam
pageBodyAttr = PageBodyAttr

--- A basic HTML web page with the default encoding.
--- @param title - the title of the page
--- @param hexps - the page's body (list of HTML expressions)
--- @return an HTML page
page :: String -> [BaseHtml] -> HtmlPage
page title hexps = HtmlPage title [PageEnc defaultEncoding] hexps

--- A standard HTML web page where the title is included
--- in the body as the first header.
--- @param title - the title of the page
--- @param hexps - the page's body (list of HTML expressions)
--- @return an HTML page with the title as the first header
headerPage :: String -> [BaseHtml] -> HtmlPage
headerPage title hexps = page title (h1 [htxt title] : hexps)

--- Adds a parameter to an HTML page.
--- @param page - a page
--- @param param - a page's parameter
--- @return an HTML page
addPageParam :: HtmlPage -> PageParam -> HtmlPage
addPageParam (HtmlPage title params hexps) param =
  HtmlPage title (param:params) hexps
addPageParam hexp@(HtmlAnswer _ _) _ = hexp

--- Adds simple cookie to an HTML page.
--- The cookies are sent to the client's browser together with this page.
--- @param cs - the cookies as a list of name/value pairs
--- @param form - the form to add cookies to
--- @return a new HTML page
addCookies :: [(String,String)] -> HtmlPage -> HtmlPage
addCookies cs (HtmlPage title params hexps) =
  HtmlPage title (map pageCookie cs ++ params) hexps
addCookies _ (HtmlAnswer _ _) =
  error "addCookies: cannot add cookie to HTML answer"

--- Adds a HTTP header to a HTML page.
--- Headers are sent to the client's browser together with the page.
--- @param key   - the name of the HTTP header field
--- @param value - the value of the HTTP header field
--- @param page  - the page to which the header is added
--- @return a new HTML page
addHttpHeader :: String -> String -> HtmlPage -> HtmlPage
addHttpHeader key value (HtmlPage t fas hs) =
  HtmlPage t (HttpHeader key value : fas) hs
addHttpHeader _ _ (HtmlAnswer _ _) =
  error "addHttpHeader: cannot add HTTP header to HTML answer"

------------------------------------------------------------------------------
--- The possible parameters of a cookie.
data CookieParam = CookieExpire ClockTime
                 | CookieDomain String
                 | CookiePath   String
                 | CookieSecure

-- Shows the cookie in standard syntax:
formatCookie :: (String,String,[CookieParam]) -> String
formatCookie (name,value,params) =
  "Set-Cookie: " ++ name ++ "=" ++ string2urlencoded value ++
  concatMap (\p->"; "++formatCookieParam p) params

-- Formats a cookie parameter:
formatCookieParam :: CookieParam -> String
formatCookieParam (CookieExpire e) = "expires=" ++ toCookieDateString e
formatCookieParam (CookieDomain d) = "domain="  ++ d
formatCookieParam (CookiePath   p) = "path="    ++ p
formatCookieParam CookieSecure     = "secure"

-- Formats a clock time into a date string for cookies:
toCookieDateString :: ClockTime -> String
toCookieDateString time =
 let (CalendarTime y mo d h mi s tz) = toUTCTime time
  in (show d ++ "-" ++ shortMonths!!(mo-1) ++ "-" ++ show y ++ " " ++
         toTimeString (CalendarTime y mo d h mi s tz) ++ " UTC")
  where shortMonths = ["Jan","Feb","Mar","Apr","May","Jun",
                       "Jul","Aug","Sep","Oct","Nov","Dec"]


--- A textual result instead of an HTML page as a result for active web pages.
--- @param txt - the contents of the result page
--- @return an HTML answer page
answerText :: String -> HtmlPage
answerText = HtmlAnswer "text/plain"

--- A textual result instead of an HTML page as a result for active web pages
--- where the encoding is given as the first parameter.
--- @param enc - the encoding of the text(e.g., "utf-8" or "iso-8859-1")
--- @param txt - the contents of the result page
--- @return an HTML answer page
answerEncText :: String -> String -> HtmlPage
answerEncText enc = HtmlAnswer ("text/plain; charset=" ++ enc)

--- Generates a redirection page to a given URL.
--- This is implemented via the HTTP response header `Location` (see also
--- <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Location>).
--- @param url - The URL target of the redirection
--- @param page - The redirection page
redirectPage :: String -> HtmlPage
redirectPage url = addHttpHeader "Location" url $ page "Redirect" []

--- Adds expire time to given HTML page.
--- @param secs - Number of seconds before document expires
--- @param page - The page to add the header information to
expires :: Int -> HtmlPage -> HtmlPage
expires secs hpage =
  hpage `addPageParam`
    PageHeadInclude (BaseStruct "meta" [("http-equiv","expires"),
                                        ("content",show secs)] [])

------------------------------------------------------------------------------
-- some useful abbreviations:

--- Basic text as HTML expression.
--- The text may contain special HTML chars (like &lt;,&gt;,&amp;,&quot;)
--- which will be quoted so that they appear as in the parameter string.
htxt :: HTML h => String -> h
htxt s = htmlText (htmlQuote s)

--- A list of strings represented as a list of HTML expressions.
--- The strings may contain special HTML chars that will be quoted.
htxts :: HTML h => [String] -> [h]
htxts = map htxt

--- An empty HTML expression.
hempty :: HTML h => h
hempty = htmlText ""

--- Non breaking Space
nbsp :: HTML h => h
nbsp = htmlText "&nbsp;"

--- Header 1
h1 :: HTML h => [h] -> h
h1 = hStruct "h1"

--- Header 2
h2 :: HTML h => [h] -> h
h2 = hStruct "h2"

--- Header 3
h3 :: HTML h => [h] -> h
h3 = hStruct "h3"

--- Header 4
h4 :: HTML h => [h] -> h
h4 = hStruct "h4"

--- Header 5
h5 :: HTML h => [h] -> h
h5 = hStruct "h5"

--- Header 6
h6 :: HTML h => [h] -> h
h6 = hStruct "h6"

--- Paragraph
par :: HTML h => [h] -> h
par = hStruct "p"

--- Section
section :: HTML h => [h] -> h
section = hStruct "section"

--- Header
header :: HTML h => [h] -> h
header = hStruct "header"

--- Footer
footer :: HTML h => [h] -> h
footer = hStruct "footer"

--- Emphasize
emphasize :: HTML h => [h] -> h
emphasize = hStruct "em"

--- Strong (more emphasized) text.
strong :: HTML h => [h] -> h
strong = hStruct "strong"

--- Boldface
bold :: HTML h => [h] -> h
bold = hStruct "b"

--- Italic
italic :: HTML h => [h] -> h
italic = hStruct "i"

--- Navigation
nav :: HTML h => [h] -> h
nav = hStruct "nav"

--- Program code
code :: HTML h => [h] -> h
code = hStruct "code"

--- Centered text
center :: HTML h => [h] -> h
center = hStruct "center"

--- Blinking text
blink :: HTML h => [h] -> h
blink = hStruct "blink"

--- Teletype font
teletype :: HTML h => [h] -> h
teletype = hStruct "tt"

--- Unformatted input, i.e., keep spaces and line breaks and
--- don't quote special characters.
pre :: HTML h => [h] -> h
pre = hStruct "pre"

--- Verbatim (unformatted), special characters (&lt;,&gt;,&amp;,&quot;)
--- are quoted.
verbatim :: HTML h => String -> h
verbatim s = hStruct "pre" [htmlText (htmlQuote s)]

--- Address
address :: HTML h => [h] -> h
address = hStruct "address"

--- Hypertext reference
href :: HTML h => String -> [h] -> h
href ref = htmlStruct "a" [("href",ref)]

--- An anchored text with a hypertext reference inside a document.
anchor :: HTML h => String -> [h] -> h
anchor anc = htmlStruct "span" [("id",anc)]

--- Unordered list.
--- @param items - the list items where each item is a list of HTML expressions
ulist :: HTML h => [[h]] -> h
ulist items = hStruct "ul" (map litem items)

--- An unordered list with classes for the entire list and the list elements.
--- The class annotation will be ignored if it is empty.
--- @param listclass - the class for the entire list structure
--- @param itemclass - the class for the list items
--- @param items - the list items where each item is a list of HTML expressions
ulistWithClass :: HTML h => String -> String -> [[h]] -> h
ulistWithClass listclass itemclass items =
  hStruct "ul" (map litemWC items) `addClass` listclass
 where
  litemWC i = litem i `addClass` itemclass

--- An unordered list with classes for the entire list
--- individual classes for the list elements.
--- The class annotation will be ignored if it is empty.
--- @param listclass - the class for the entire list structure
--- @param classitems - the list items together with their classes
ulistWithItemClass :: HTML h => String -> [(String,[h])] -> h
ulistWithItemClass listclass classeditems =
  hStruct "ul" (map litemWC classeditems) `addClass` listclass
 where
  litemWC (c,i) = litem i `addClass` c

--- Ordered list.
--- @param items - the list items where each item is a list of HTML expressions
olist :: HTML h => [[h]] -> h
olist items = hStruct "ol" (map litem items)

--- An ordered list with classes for the entire list and the list elements.
--- The class annotation will be ignored if it is empty.
--- @param listclass - the class for the entire list structure
--- @param itemclass - the class for the list items
--- @param items - the list items where each item is a list of HTML expressions
olistWithClass :: HTML h => String -> String -> [[h]] -> h
olistWithClass listclass itemclass items =
  hStruct "ol" (map litemWC items) `addClass` listclass
 where
  litemWC i = litem i `addClass` itemclass

--- An ordered list with classes for the entire list
--- individual classes for the list elements.
--- The class annotation will be ignored if it is empty.
--- @param listclass - the class for the entire list structure
--- @param classitems - the list items together with their classes
olistWithItemClass :: HTML h => String -> [(String,[h])] -> h
olistWithItemClass listclass classeditems =
  hStruct "ol" (map litemWC classeditems) `addClass` listclass
 where
  litemWC (c,i) = litem i `addClass` c

--- A single list item (usually not explicitly used)
litem :: HTML h => [h] -> h
litem = hStruct "li"

--- Description list
--- @param items - a list of (title/description) pairs (of HTML expressions)
dlist :: HTML h => [([h],[h])] -> h
dlist items = hStruct "dl" (concatMap ditem items)
 where
  ditem (hexps1,hexps2) = [htmlStruct "dt" [] hexps1,
                           htmlStruct "dd" [] hexps2]

--- Table with a matrix of items where each item is a list of HTML expressions.
table :: HTML h => [[[h]]] -> h
table = hStruct "table" . map (\row -> hStruct "tr" (map (hStruct "td") row))

--- Table with a matrix of items (each item is a list of HTML expressions)
--- with classes for the entire table, each row, and each data element.
--- The class annotation will be ignored if it is empty.
--- @param tableclass - the class for the entire table structure
--- @param rowclass   - the class for the table rows
--- @param dataclass  - the class for the table data items
--- @param items - the matrix of table items where each item is a
---                list of HTML expressions
tableWithClass :: HTML h => String -> String -> String -> [[[h]]] -> h
tableWithClass tableclass rowclass dataclass items =
 hStruct "table"
   (map (\row -> hStruct "tr"
                   (map (\d -> hStruct "td" d `addClass` dataclass) row)
                   `addClass` rowclass)
        items) `addClass` tableclass

--- Similar to `table` but introduces header tags for the first row.
headedTable :: HTML h => [[[h]]] -> h
headedTable = hStruct "table" . headedItems
 where
  headedItems []         = []
  headedItems (row:rows) = hStruct "tr" (map (hStruct "th") row) :
                           map (\r -> hStruct "tr" (map (hStruct "td") r)) rows

--- Horizontal rule
hrule :: HTML h => h
hrule = hStruct "hr" []

--- Break a line
breakline :: HTML h => h
breakline = hStruct "br" []

--- Image
--- @param src - the URL of the image
--- @param alt - the alternative text shown instead of the image
image :: HTML h => String -> String -> h
image src alt = htmlStruct "img" [("src",src),("alt",htmlQuote alt)] []


-------------- styles and document structuring:
--- Defines a style sheet to be used in this HTML document.
--- @param css - a string in CSS format
styleSheet :: HTML h => String -> h
styleSheet css = htmlStruct "style" [("type","text/css")] [htmlText css]

--- Provides a style for HTML elements.
--- The style argument is the name of a style class defined in a
--- style definition (see `styleSheet`) or in an external
--- style sheet (see form and page parameters `FormCSS` and `PageCSS`).
--- @param st - name of a style class
--- @param hexps - list of HTML expressions
style :: HTML h => String -> [h] -> h
style st = htmlStruct "span" [("class",st)]

--- Provides a style for a basic text.
--- The style argument is the name of a style class defined in an
--- external style sheet.
--- @param st - name of a style class
--- @param txt - a string (special characters will be quoted)
textstyle :: HTML h => String -> String -> h
textstyle st txt = htmlStruct "span" [("class",st)] [htxt txt]

--- Provides a style for a block of HTML elements.
--- The style argument is the name of a style class defined in an
--- external style sheet. This element is used (in contrast to "style")
--- for larger blocks of HTML elements since a line break is placed
--- before and after these elements.
--- @param st - name of a style class
--- @param hexps - list of HTML expressions
blockstyle :: HTML h => String -> [h] -> h
blockstyle st = htmlStruct "div" [("class",st)]

--- Joins a list of HTML elements into a single HTML element.
--- Although this construction has no rendering, it is sometimes useful
--- for programming when several HTML elements must be put together.
--- @param hexps - list of HTML expressions
inline :: HTML h => [h] -> h
inline = hStruct "span"

--- Joins a list of HTML elements into a block.
--- A line break is placed before and after these elements.
--- @param hexps - list of HTML expressions
block :: HTML h => [h] -> h
block = hStruct "div"

--- A hidden field to pass a value referenced by a fixed name.
--- This function should be used with care since it may cause
--- conflicts with the CGI-based implementation of this library.
hiddenField :: HTML h => String -> String -> h
hiddenField name value =
  htmlStruct "input" [("type","hidden"),("name",name),("value",value)] []

------------------------------------------------------------------------------
-- Forms and input fields:

--- A form embedded in an HTML expression.
--- The parameter is a form defined as an exported top-level operation
--- in the CGI program so that it can be accessed by the main program.
--- The URL of the generated form is the same as the main page, i.e.,
--- the current URL parameter is passed to the form (which is
--- useful for REST-based programming with URL parameters).
--- The form uses a hidden field named `FORMID` to identify the form
--- in the submitted form controller.
--- 
--- Since form elements can not be nested, see
--- [HTML](https://html.spec.whatwg.org/multipage/forms.html#the-form-element),
--- the form element itself is a static HTML expression.
formElem :: HtmlFormDef a -> BaseHtml
formElem formspec = BaseAction formAction
 where
  formAction = do
    urlparam <- getUrlParameter
    he       <- genInitForm formspec
    return $
       HtmlStruct "form" [("method", "post"), ("action", '?' : urlparam)]
         (hiddenField "FORMID" (formDefId formspec) : fst (instHtmlRefs he 0))

--- A button to submit a form with a label string and an event handler.
button :: String -> HtmlHandler -> HtmlExp
button label handler
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlEvent cref handler
              (HtmlStruct "input" [("type","submit"), ("name",ref),
                                   ("value",htmlQuote label)] [])
 where
  cref,ref free

--- Reset button with a label string
resetButton :: String -> HtmlExp
resetButton label =
    HtmlStruct "input" [("type","reset"),("value",htmlQuote label)] []

--- Submit button in form of an imag.
--- @param src - url of the image
--- @param handler - event handler
imageButton :: String -> HtmlHandler -> HtmlExp
imageButton src handler
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlEvent cref handler
       (HtmlStruct "input" [("type","image"),("name",ref),("src",src)] [])
 where
  cref,ref free

--- Input text field with a reference and an initial contents
textField :: HtmlRef -> String -> HtmlExp
textField cref contents
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlInput cref
      (HtmlStruct "input" [("type","text"),("name",ref),
                           ("value",htmlQuote contents)] [])
 where ref free

--- Input text field (where the entered text is obscured) with a reference
password :: HtmlRef -> HtmlExp
password cref
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlInput cref
       (HtmlStruct "input" [("type","password"),("name",ref)] [])
 where
   ref free

--- Input text area with a reference, height/width, and initial contents
textArea :: HtmlRef -> (Int,Int) -> String -> HtmlExp
textArea cref (height,width) contents
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlInput cref
       (HtmlStruct "textarea" [("name",ref),
                                ("rows",show height),("cols",show width)]
                               [htxt contents])
 where
   ref free

--- A checkbox with a reference and a value.
--- The value is returned if checkbox is on, otherwise "" is returned.
checkBox :: HtmlRef -> String -> HtmlExp
checkBox cref value
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlInput cref
       (HtmlStruct "input" [("type","checkbox"),("name",ref),
                            ("value",htmlQuote value)] [])
 where
   ref free

--- A checkbox that is initially checked with a reference and a value.
--- The value is returned if checkbox is on, otherwise "" is returned.
checkedBox :: HtmlRef -> String -> HtmlExp
checkedBox cref value
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlInput cref
       (HtmlStruct "input" [("type","checkbox"),("name",ref),
                            ("value",htmlQuote value),("checked","checked")] [])
 where
   ref free

--- A main button of a radio (initially "on") with a reference and a value.
--- The value is returned of this button is on.
--- A complete radio button suite always consists of a main button
--- (radio_main) and some further buttons (radio_others) with the
--- same reference. Initially, the main button is selected
--- (or nothing is selected if one uses radio_main_off instead of radio_main).
--- The user can select another button but always at most one button
--- of the radio can be selected. The value corresponding to the
--- selected button is returned in the environment for this radio reference.
radioMain :: HtmlRef -> String -> HtmlExp
radioMain cref value
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlInput cref
       (HtmlStruct "input" [("type","radio"),("name",ref),
                            ("value",htmlQuote value),("checked","yes")] [])
 where
   ref free

--- A main button of a radio (initially "off") with a reference and a value.
--- The value is returned of this button is on.
radioMainOff :: HtmlRef -> String -> HtmlExp
radioMainOff cref value
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlInput cref
       (HtmlStruct "input" [("type","radio"),("name",ref),
                            ("value",htmlQuote value)] [])
 where
   ref free

--- A further button of a radio (initially "off") with a reference (identical
--- to the main button of this radio) and a value.
--- The value is returned of this button is on.
radioOther :: HtmlRef -> String -> HtmlExp
radioOther cref value
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlStruct "input"
               [("type","radio"),("name",ref),("value",htmlQuote value)] []
 where
   ref free

--- A selection button with a reference and a list of name/value pairs.
--- The names are shown in the selection and the value is returned
--- for the selected name.
selection :: HtmlRef -> [(String,String)] -> HtmlExp
selection cref menue
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlInput cref
       (HtmlStruct "select" [("name",ref)]
         ((concat . map (\(n,v)->[HtmlStruct "option" [("value",v)] [htxt n]]))
          menue))
 where
   ref free

--- A selection button with a reference, a list of name/value pairs,
--- and a preselected item in this list.
--- The names are shown in the selection and the value is returned
--- for the selected name.
--- @param ref - a CGI reference
--- @param nvs - list of name/value pairs
--- @param sel - the index of the initially selected item in the list nvs
--- @return an HTML expression representing the selection button
selectionInitial :: HtmlRef -> [(String,String)] -> Int -> HtmlExp
selectionInitial cref sellist sel
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlInput cref (HtmlStruct "select" [("name",ref)] (selOption sellist sel))
 where
   ref free

   selOption [] _ = []
   selOption ((n,v):nvs) i =
      HtmlStruct "option"
        ([("value",v)] ++ if i==0 then [("selected","selected")] else [])
        [htxt n] : selOption nvs (i-1)

--- A selection button with a reference and a list of name/value/flag pairs.
--- The names are shown in the selection and the value is returned
--- if the corresponding name is selected. If flag is True, the
--- corresonding name is initially selected. If more than one name
--- has been selected, all values are returned in one string
--- where the values are separated by newline (`'\n'`) characters.
multipleSelection :: HtmlRef -> [(String,String,Bool)] -> HtmlExp
multipleSelection cref sellist
  | cref =:= HtmlRef ref -- instantiate cref argument
  = HtmlInput cref
             (HtmlStruct "select" [("name",ref),("multiple","multiple")]
                                   (map selOption sellist))
 where
   ref free

   selOption (n,v,flag) =
      HtmlStruct "option"
        ([("value",v)] ++ if flag then [("selected","selected")] else [])
        [htxt n]

------------------------------------------------------------------------------
--- Quotes special characters (`<`,`>`,`&`,`"`, umlauts) in a string
--- as HTML special characters.
htmlQuote :: String -> String
htmlQuote [] = []
htmlQuote (c:cs) | c=='<' = "&lt;"   ++ htmlQuote cs
                 | c=='>' = "&gt;"   ++ htmlQuote cs
                 | c=='&' = "&amp;"  ++ htmlQuote cs
                 | c=='"' = "&quot;" ++ htmlQuote cs
                 | otherwise = htmlIsoUmlauts [c] ++ htmlQuote cs

--- Translates umlauts in iso-8859-1 encoding into HTML special characters.
htmlIsoUmlauts :: String -> String
htmlIsoUmlauts [] = []
htmlIsoUmlauts (c:cs) | oc==228 = "&auml;"  ++ htmlIsoUmlauts cs
                      | oc==246 = "&ouml;"  ++ htmlIsoUmlauts cs
                      | oc==252 = "&uuml;"  ++ htmlIsoUmlauts cs
                      | oc==196 = "&Auml;"  ++ htmlIsoUmlauts cs
                      | oc==214 = "&Ouml;"  ++ htmlIsoUmlauts cs
                      | oc==220 = "&Uuml;"  ++ htmlIsoUmlauts cs
                      | oc==223 = "&szlig;" ++ htmlIsoUmlauts cs
                      | oc==197 = "&Aring;" ++ htmlIsoUmlauts cs
                      | oc==250 = "&uacute;"++ htmlIsoUmlauts cs
                      | oc==237 = "&iacute;"++ htmlIsoUmlauts cs
                      | oc==225 = "&aacute;"++ htmlIsoUmlauts cs
                      | otherwise = c : htmlIsoUmlauts cs
  where oc = ord c

------------------------------------------------------------------------------
--- Adds an attribute (name/value pair) to an HTML element.
addAttr :: HTML h => h -> (String,String) -> h
addAttr hexp attr = addAttrs hexp [attr]

--- Adds a list of attributes (name/value pair) to an HTML element.
addAttrs :: HTML h => h -> Attrs -> h
addAttrs h newattrs = updAttrs (++newattrs) h

--- Adds a class attribute to an HTML element
--- (if the class attribute is not empty).
addClass :: HTML h => h -> String -> h
addClass hexp cls | null cls  = hexp
                  | otherwise = addAttr hexp ("class",cls)

------------------------------------------------------------------------------
--- Transforms a list of HTML expressions into string representation.
showBaseHtmls :: [BaseHtml] -> String
showBaseHtmls hexps = showsBaseHtmls 0 hexps ""

-- is this a tag where a line break can be safely added?
tagWithLn :: String -> Bool
tagWithLn t = t/="" &&
              t `elem` ["br","p","li","ul","ol","dl","dt","dd","hr",
                        "h1","h2","h3","h4","h5","h6","div",
                        "html","title","head","body","link","meta","script",
                        "form","table","tr","td"]

--- Transforms a single HTML expression into string representation.
showBaseHtml :: BaseHtml -> String
showBaseHtml hexp = showsBaseHtml 0 hexp ""

--- HTML tags that have no end tag in HTML:
noEndTags :: [String]
noEndTags = ["img","input","link","meta"]

showsBaseHtml :: Int -> BaseHtml -> ShowS
showsBaseHtml _ (BaseText s)                 = showString s
showsBaseHtml i (BaseStruct tag attrs hexps) =
  let maybeLn j = if tagWithLn tag then nl . showTab j else id
   in maybeLn i .
      (if null hexps && (null attrs || tag `elem` noEndTags)
         then showsHtmlOpenTag tag attrs "/>"
         else showsHtmlOpenTag tag attrs ">" . maybeLn (i+2) . showExps hexps .
              maybeLn i . showString "</" . showString tag . showChar '>'
      ) . maybeLn i
 where
  showExps = if tag=="pre"
               then concatS . map (showsBaseHtml 0)
               else showsBaseHtmls (i+2)
showsBaseHtml _ (BaseAction  _)              =
  error "HTML.Base.showsBaseHtml: BaseAction occurred"

showsBaseHtmls :: Int -> [BaseHtml] -> ShowS
showsBaseHtmls _ []       = id
showsBaseHtmls i (he:hes) = showsWithLnPrefix he . showsBaseHtmls i hes
 where
  showsWithLnPrefix hexp = let s = textOfBaseHtml hexp
                           in if s /= "" && isSpace (head s)
                                then nl . showTab i . showString (tail s)
                                else showsBaseHtml i hexp

  -- get the string contents of an HTML expression:
  textOfBaseHtml :: BaseHtml -> String
  textOfBaseHtml (BaseText   s)        = s
  textOfBaseHtml (BaseStruct _ _ _)    = ""
  textOfBaseHtml (BaseAction _)        = ""

showTab :: Int -> ShowS
showTab n = showString (take n (repeat ' '))

showsHtmlOpenTag :: String -> Attrs -> String -> ShowS
showsHtmlOpenTag tag attrs close =
  showChar '<' . showString tag .
  concatS (map attr2string attrs) . showString close
 where
    attr2string (attr,value) =
      showChar ' ' . showString attr .
                     showString "=\"" . encodeQuotes value . showChar '"'

    -- encode double quotes as "&quot;":
    encodeQuotes []                 = id
    encodeQuotes (c:cs) | c=='"'    = showString "&quot;" . encodeQuotes cs
                        | otherwise = showChar c . encodeQuotes cs

nl :: ShowS
nl = showChar '\n'

concatS :: [ShowS] -> ShowS
concatS []       = id
concatS xs@(_:_) = foldr1 (\ f g -> f . g) xs

------------------------------------------------------------------------------
--- Transforms HTML page into string representation.
--- @param page - the HTML page
--- @return string representation of the HTML document
showHtmlPage :: HtmlPage -> String
showHtmlPage (HtmlAnswer _ cont)            = cont
showHtmlPage (HtmlPage   title params html) =
  htmlPrelude ++
  showBaseHtml (BaseStruct "html" htmlTagAttrs
                  [BaseStruct "head" []
                       ([BaseStruct "title" [] [BaseText (htmlQuote title)]] ++
                        concatMap pageParam2HTML params),
                   BaseStruct "body" bodyattrs html])
 where
  bodyattrs = [attr | (PageBodyAttr attr) <- params]

--- Translates page parameters into HTML expressions.
--- Used to show HTML pages.
pageParam2HTML :: PageParam -> [BaseHtml]
pageParam2HTML (PageEnc enc) =
   [BaseStruct "meta" [("http-equiv","Content-Type"),
                       ("content","text/html; charset="++enc)] []]
pageParam2HTML (PageCookie _ _ _) = [] -- cookies are differently processed
pageParam2HTML (PageCSS css) =
   [BaseStruct "link" [("rel","stylesheet"),("type","text/css"),("href",css)]
               []]
pageParam2HTML (HttpHeader _ _) = [] -- page headers are differently processed
pageParam2HTML (PageJScript js) =
   [BaseStruct "script" [("type","text/javascript"),("src",js)] []]
pageParam2HTML (PageMeta attrs) = [BaseStruct "meta" attrs []]
pageParam2HTML (PageLink attrs) = [BaseStruct "link" attrs []]
pageParam2HTML (PageHeadInclude hexp) = [hexp]
pageParam2HTML (PageBodyAttr _) = [] --these attributes are separately processed

--- Standard header for generated HTML pages.
htmlPrelude :: String
htmlPrelude = "<!DOCTYPE html>\n"

--- Standard attributes for element "html".
htmlTagAttrs :: Attrs
htmlTagAttrs = [("lang","en")]

------------------------------------------------------------------------------
--- Gets the parameter attached to the URL of the script.
--- For instance, if the script is called with URL
--- "http://.../script.cgi?parameter", then "parameter" is
--- returned by this I/O action.
--- Note that an URL parameter should be "URL encoded" to avoid
--- the appearance of characters with a special meaning.
--- Use the functions "urlencoded2string" and "string2urlencoded"
--- to decode and encode such parameters, respectively.

getUrlParameter :: IO String
getUrlParameter = getEnv "QUERY_STRING"

--- Translates an URL encoded string into equivalent ASCII string.
urlencoded2string :: String -> String
urlencoded2string [] = []
urlencoded2string (c:cs)
  | c == '+'  = ' ' : urlencoded2string cs
  | c == '%'  = chr (case readHex (take 2 cs) of [(n,"")] -> n
                                                 _        -> 0)
                 : urlencoded2string (drop 2 cs)
  | otherwise = c : urlencoded2string cs

--- Translates arbitrary strings into equivalent URL encoded strings.
string2urlencoded :: String -> String
string2urlencoded [] = []
string2urlencoded (c:cs)
  | isAlphaNum c = c : string2urlencoded cs
  | c == ' '     = '+' : string2urlencoded cs
  | otherwise = let oc = ord c
    in '%' : int2hex(oc `div` 16) : int2hex(oc `mod` 16) : string2urlencoded cs
 where
   int2hex i = if i<10 then chr (ord '0' + i)
                       else chr (ord 'A' + i - 10)


------------------------------------------------------------------------------
--- Gets the cookies sent from the browser for the current CGI script.
--- The cookies are represented in the form of name/value pairs since
--- no other components are important here.
getCookies :: IO [(String,String)]
getCookies = do
  cookiestring <- getEnv "HTTP_COOKIE"
  return $ parseCookies cookiestring

-- translate a string of cookies (of the form "NAME1=VAL1; NAME2=VAL")
-- into a list of name/value pairs:
parseCookies :: String -> [(String,String)]
parseCookies str =
  if null str
    then []
    else let (c1,cs) = break (==';') str
         in parseCookie c1 :
            parseCookies (dropWhile (==' ') (if cs=="" then "" else tail cs))
 where
  parseCookie s = let (name,evalue) = break (=='=') s in
      (name, if evalue=="" then "" else urlencoded2string (tail evalue))

--- For image buttons: retrieve the coordinates where the user clicked
--- within the image.
coordinates :: HtmlEnv -> Maybe (Int,Int)
coordinates env = let x = env (HtmlRef "x")
                      y = env (HtmlRef "y")
                  in if x/="" && y/=""
                       then Just (tryReadNat 0 x, tryReadNat 0 y)
                       else Nothing
 where
  tryReadNat d s = case readNat s of [(i,"")] -> i
                                     _        -> d

------------------------------------------------------------------------------