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date Fri, 11 Oct 2024 22:40:23 +0000
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1 <!doctype html>
2
3 <title>CodeMirror: Haskell-literate mode</title>
4 <meta charset="utf-8"/>
5 <link rel=stylesheet href="../../doc/docs.css">
6
7 <link rel="stylesheet" href="../../lib/codemirror.css">
8 <script src="../../lib/codemirror.js"></script>
9 <script src="haskell-literate.js"></script>
10 <script src="../haskell/haskell.js"></script>
11 <style>.CodeMirror {
12 border-top : 1px solid #DDDDDD;
13 border-bottom : 1px solid #DDDDDD;
14 }</style>
15 <div id=nav>
16 <a href="https://codemirror.net/5"><h1>CodeMirror</h1><img id=logo
17 src="../../doc/logo.png"></a>
18
19 <ul>
20 <li><a href="../../index.html">Home</a>
21 <li><a href="../../doc/manual.html">Manual</a>
22 <li><a href="https://github.com/codemirror/codemirror5">Code</a>
23 </ul>
24 <ul>
25 <li><a href="../index.html">Language modes</a>
26 <li><a class=active href="#">Haskell-literate</a>
27 </ul>
28 </div>
29
30 <article>
31 <h2>Haskell literate mode</h2>
32 <form>
33 <textarea id="code" name="code">
34 > {-# LANGUAGE OverloadedStrings #-}
35 > {-# OPTIONS_GHC -fno-warn-unused-do-bind #-}
36 > import Control.Applicative ((<$>), (<*>))
37 > import Data.Maybe (isJust)
38
39 > import Data.Text (Text)
40 > import Text.Blaze ((!))
41 > import qualified Data.Text as T
42 > import qualified Happstack.Server as Happstack
43 > import qualified Text.Blaze.Html5 as H
44 > import qualified Text.Blaze.Html5.Attributes as A
45
46 > import Text.Digestive
47 > import Text.Digestive.Blaze.Html5
48 > import Text.Digestive.Happstack
49 > import Text.Digestive.Util
50
51 Simple forms and validation
52 ---------------------------
53
54 Let's start by creating a very simple datatype to represent a user:
55
56 > data User = User
57 > { userName :: Text
58 > , userMail :: Text
59 > } deriving (Show)
60
61 And dive in immediately to create a `Form` for a user. The `Form v m a` type
62 has three parameters:
63
64 - `v`: the type for messages and errors (usually a `String`-like type, `Text` in
65 this case);
66 - `m`: the monad we are operating in, not specified here;
67 - `a`: the return type of the `Form`, in this case, this is obviously `User`.
68
69 > userForm :: Monad m => Form Text m User
70
71 We create forms by using the `Applicative` interface. A few form types are
72 provided in the `Text.Digestive.Form` module, such as `text`, `string`,
73 `bool`...
74
75 In the `digestive-functors` library, the developer is required to label each
76 field using the `.:` operator. This might look like a bit of a burden, but it
77 allows you to do some really useful stuff, like separating the `Form` from the
78 actual HTML layout.
79
80 > userForm = User
81 > <$> "name" .: text Nothing
82 > <*> "mail" .: check "Not a valid email address" checkEmail (text Nothing)
83
84 The `check` function enables you to validate the result of a form. For example,
85 we can validate the email address with a really naive `checkEmail` function.
86
87 > checkEmail :: Text -> Bool
88 > checkEmail = isJust . T.find (== '@')
89
90 More validation
91 ---------------
92
93 For our example, we also want descriptions of Haskell libraries, and in order to
94 do that, we need package versions...
95
96 > type Version = [Int]
97
98 We want to let the user input a version number such as `0.1.0.0`. This means we
99 need to validate if the input `Text` is of this form, and then we need to parse
100 it to a `Version` type. Fortunately, we can do this in a single function:
101 `validate` allows conversion between values, which can optionally fail.
102
103 `readMaybe :: Read a => String -> Maybe a` is a utility function imported from
104 `Text.Digestive.Util`.
105
106 > validateVersion :: Text -> Result Text Version
107 > validateVersion = maybe (Error "Cannot parse version") Success .
108 > mapM (readMaybe . T.unpack) . T.split (== '.')
109
110 A quick test in GHCi:
111
112 ghci> validateVersion (T.pack "0.3.2.1")
113 Success [0,3,2,1]
114 ghci> validateVersion (T.pack "0.oops")
115 Error "Cannot parse version"
116
117 It works! This means we can now easily add a `Package` type and a `Form` for it:
118
119 > data Category = Web | Text | Math
120 > deriving (Bounded, Enum, Eq, Show)
121
122 > data Package = Package Text Version Category
123 > deriving (Show)
124
125 > packageForm :: Monad m => Form Text m Package
126 > packageForm = Package
127 > <$> "name" .: text Nothing
128 > <*> "version" .: validate validateVersion (text (Just "0.0.0.1"))
129 > <*> "category" .: choice categories Nothing
130 > where
131 > categories = [(x, T.pack (show x)) | x <- [minBound .. maxBound]]
132
133 Composing forms
134 ---------------
135
136 A release has an author and a package. Let's use this to illustrate the
137 composability of the digestive-functors library: we can reuse the forms we have
138 written earlier on.
139
140 > data Release = Release User Package
141 > deriving (Show)
142
143 > releaseForm :: Monad m => Form Text m Release
144 > releaseForm = Release
145 > <$> "author" .: userForm
146 > <*> "package" .: packageForm
147
148 Views
149 -----
150
151 As mentioned before, one of the advantages of using digestive-functors is
152 separation of forms and their actual HTML layout. In order to do this, we have
153 another type, `View`.
154
155 We can get a `View` from a `Form` by supplying input. A `View` contains more
156 information than a `Form`, it has:
157
158 - the original form;
159 - the input given by the user;
160 - any errors that have occurred.
161
162 It is this view that we convert to HTML. For this tutorial, we use the
163 [blaze-html] library, and some helpers from the `digestive-functors-blaze`
164 library.
165
166 [blaze-html]: http://jaspervdj.be/blaze/
167
168 Let's write a view for the `User` form. As you can see, we here refer to the
169 different fields in the `userForm`. The `errorList` will generate a list of
170 errors for the `"mail"` field.
171
172 > userView :: View H.Html -> H.Html
173 > userView view = do
174 > label "name" view "Name: "
175 > inputText "name" view
176 > H.br
177 >
178 > errorList "mail" view
179 > label "mail" view "Email address: "
180 > inputText "mail" view
181 > H.br
182
183 Like forms, views are also composable: let's illustrate that by adding a view
184 for the `releaseForm`, in which we reuse `userView`. In order to do this, we
185 take only the parts relevant to the author from the view by using `subView`. We
186 can then pass the resulting view to our own `userView`.
187 We have no special view code for `Package`, so we can just add that to
188 `releaseView` as well. `childErrorList` will generate a list of errors for each
189 child of the specified form. In this case, this means a list of errors from
190 `"package.name"` and `"package.version"`. Note how we use `foo.bar` to refer to
191 nested forms.
192
193 > releaseView :: View H.Html -> H.Html
194 > releaseView view = do
195 > H.h2 "Author"
196 > userView $ subView "author" view
197 >
198 > H.h2 "Package"
199 > childErrorList "package" view
200 >
201 > label "package.name" view "Name: "
202 > inputText "package.name" view
203 > H.br
204 >
205 > label "package.version" view "Version: "
206 > inputText "package.version" view
207 > H.br
208 >
209 > label "package.category" view "Category: "
210 > inputSelect "package.category" view
211 > H.br
212
213 The attentive reader might have wondered what the type parameter for `View` is:
214 it is the `String`-like type used for e.g. error messages.
215 But wait! We have
216 releaseForm :: Monad m => Form Text m Release
217 releaseView :: View H.Html -> H.Html
218 ... doesn't this mean that we need a `View Text` rather than a `View Html`? The
219 answer is yes -- but having `View Html` allows us to write these views more
220 easily with the `digestive-functors-blaze` library. Fortunately, we will be able
221 to fix this using the `Functor` instance of `View`.
222 fmap :: Monad m => (v -> w) -> View v -> View w
223 A backend
224 ---------
225 To finish this tutorial, we need to be able to actually run this code. We need
226 an HTTP server for that, and we use [Happstack] for this tutorial. The
227 `digestive-functors-happstack` library gives about everything we need for this.
228 [Happstack]: http://happstack.com/
229
230 > site :: Happstack.ServerPart Happstack.Response
231 > site = do
232 > Happstack.decodeBody $ Happstack.defaultBodyPolicy "/tmp" 4096 4096 4096
233 > r <- runForm "test" releaseForm
234 > case r of
235 > (view, Nothing) -> do
236 > let view' = fmap H.toHtml view
237 > Happstack.ok $ Happstack.toResponse $
238 > template $
239 > form view' "/" $ do
240 > releaseView view'
241 > H.br
242 > inputSubmit "Submit"
243 > (_, Just release) -> Happstack.ok $ Happstack.toResponse $
244 > template $ do
245 > css
246 > H.h1 "Release received"
247 > H.p $ H.toHtml $ show release
248 >
249 > main :: IO ()
250 > main = Happstack.simpleHTTP Happstack.nullConf site
251
252 Utilities
253 ---------
254
255 > template :: H.Html -> H.Html
256 > template body = H.docTypeHtml $ do
257 > H.head $ do
258 > H.title "digestive-functors tutorial"
259 > css
260 > H.body body
261 > css :: H.Html
262 > css = H.style ! A.type_ "text/css" $ do
263 > "label {width: 130px; float: left; clear: both}"
264 > "ul.digestive-functors-error-list {"
265 > " color: red;"
266 > " list-style-type: none;"
267 > " padding-left: 0px;"
268 > "}"
269 </textarea>
270 </form>
271
272 <p><strong>MIME types
273 defined:</strong> <code>text/x-literate-haskell</code>.</p>
274
275 <p>Parser configuration parameters recognized: <code>base</code> to
276 set the base mode (defaults to <code>"haskell"</code>).</p>
277
278 <script>
279 var editor = CodeMirror.fromTextArea(document.getElementById("code"), {mode: "haskell-literate"});
280 </script>
281
282 </article>