520 lines
13 KiB
Text
520 lines
13 KiB
Text
# Generating Javascript functions to query an API
|
|
|
|
We will now see how **servant** lets you turn an API type into javascript
|
|
functions that you can call to query a webservice.
|
|
|
|
For this, we will consider a simple page divided in two parts. At the top, we
|
|
will have a search box that lets us search in a list of Haskell books by
|
|
author/title with a list of results that gets updated every time we enter or
|
|
remove a character, while at the bottom we will be able to see the classical
|
|
[probabilistic method to approximate
|
|
pi](http://en.wikipedia.org/wiki/Approximations_of_%CF%80#Summing_a_circle.27s_area),
|
|
using a webservice to get random points. Finally, we will serve an HTML file
|
|
along with a couple of Javascript files, among which one that's automatically
|
|
generated from the API type and which will provide ready-to-use functions to
|
|
query your API.
|
|
|
|
The source for this tutorial section is a literate haskell file, so first we
|
|
need to have some language extensions and imports:
|
|
|
|
``` haskell
|
|
{-# LANGUAGE DataKinds #-}
|
|
{-# LANGUAGE DeriveGeneric #-}
|
|
{-# LANGUAGE OverloadedStrings #-}
|
|
{-# LANGUAGE TypeOperators #-}
|
|
|
|
module Javascript where
|
|
|
|
import Control.Monad.IO.Class
|
|
import Data.Aeson
|
|
import Data.Proxy
|
|
import Data.Text as T (Text)
|
|
import Data.Text.IO as T (writeFile, readFile)
|
|
import GHC.Generics
|
|
import Language.Javascript.JQuery
|
|
import Network.Wai
|
|
import Network.Wai.Handler.Warp
|
|
import qualified Data.Text as T
|
|
import Servant
|
|
import Servant.JS
|
|
import System.Random
|
|
```
|
|
|
|
Now let's have the API type(s) and the accompanying datatypes.
|
|
|
|
``` haskell
|
|
type API = "point" :> Get '[JSON] Point
|
|
:<|> "books" :> QueryParam "q" Text :> Get '[JSON] (Search Book)
|
|
:<|> EmptyAPI
|
|
|
|
type API' = API :<|> Raw
|
|
|
|
data Point = Point
|
|
{ x :: Double
|
|
, y :: Double
|
|
} deriving Generic
|
|
|
|
instance ToJSON Point
|
|
|
|
data Search a = Search
|
|
{ query :: Text
|
|
, results :: [a]
|
|
} deriving Generic
|
|
|
|
mkSearch :: Text -> [a] -> Search a
|
|
mkSearch = Search
|
|
|
|
instance ToJSON a => ToJSON (Search a)
|
|
|
|
data Book = Book
|
|
{ author :: Text
|
|
, title :: Text
|
|
, year :: Int
|
|
} deriving Generic
|
|
|
|
instance ToJSON Book
|
|
|
|
book :: Text -> Text -> Int -> Book
|
|
book = Book
|
|
```
|
|
|
|
We need a "book database". For the purpose of this guide, let's restrict
|
|
ourselves to the following books.
|
|
|
|
``` haskell
|
|
books :: [Book]
|
|
books =
|
|
[ book "Paul Hudak" "The Haskell School of Expression: Learning Functional Programming through Multimedia" 2000
|
|
, book "Bryan O'Sullivan, Don Stewart, and John Goerzen" "Real World Haskell" 2008
|
|
, book "Miran Lipovača" "Learn You a Haskell for Great Good!" 2011
|
|
, book "Graham Hutton" "Programming in Haskell" 2007
|
|
, book "Simon Marlow" "Parallel and Concurrent Programming in Haskell" 2013
|
|
, book "Richard Bird" "Introduction to Functional Programming using Haskell" 1998
|
|
]
|
|
```
|
|
|
|
Now, given an optional search string `q`, we want to perform a case insensitive
|
|
search in that list of books. We're obviously not going to try and implement
|
|
the best possible algorithm, this is out of scope for this tutorial. The
|
|
following simple linear scan will do, given how small our list is.
|
|
|
|
``` haskell
|
|
searchBook :: Monad m => Maybe Text -> m (Search Book)
|
|
searchBook Nothing = return (mkSearch "" books)
|
|
searchBook (Just q) = return (mkSearch q books')
|
|
|
|
where books' = filter (\b -> q' `T.isInfixOf` T.toLower (author b)
|
|
|| q' `T.isInfixOf` T.toLower (title b)
|
|
)
|
|
books
|
|
q' = T.toLower q
|
|
```
|
|
|
|
We also need an endpoint that generates random points `(x, y)` with `-1 <= x,y
|
|
<= 1`. The code below uses
|
|
[random](http://hackage.haskell.org/package/random)'s `System.Random`.
|
|
|
|
``` haskell
|
|
randomPoint :: MonadIO m => m Point
|
|
randomPoint = liftIO . getStdRandom $ \g ->
|
|
let (rx, g') = randomR (-1, 1) g
|
|
(ry, g'') = randomR (-1, 1) g'
|
|
in (Point rx ry, g'')
|
|
```
|
|
|
|
If we add static file serving, our server is now complete.
|
|
|
|
``` haskell
|
|
api :: Proxy API
|
|
api = Proxy
|
|
|
|
api' :: Proxy API'
|
|
api' = Proxy
|
|
|
|
server :: Server API
|
|
server = randomPoint
|
|
:<|> searchBook
|
|
:<|> emptyAPIServer
|
|
|
|
server' :: Server API'
|
|
server' = server
|
|
:<|> serveDirectoryFileServer "static"
|
|
|
|
app :: Application
|
|
app = serve api' server'
|
|
|
|
main :: IO ()
|
|
main = run 8000 app
|
|
```
|
|
|
|
Why two different API types, proxies and servers though? Simply because we
|
|
don't want to generate javascript functions for the `Raw` part of our API type,
|
|
so we need a `Proxy` for our API type `API'` without its `Raw` endpoint.
|
|
|
|
Very similarly to how one can derive haskell functions, we can derive the
|
|
javascript with just a simple function call to `jsForAPI` from
|
|
`Servant.JS`.
|
|
|
|
``` haskell
|
|
apiJS1 :: Text
|
|
apiJS1 = jsForAPI api jquery
|
|
```
|
|
|
|
This `Text` contains 2 Javascript functions, 'getPoint' and 'getBooks':
|
|
|
|
``` javascript
|
|
|
|
var getPoint = function(onSuccess, onError)
|
|
{
|
|
$.ajax(
|
|
{ url: '/point'
|
|
, success: onSuccess
|
|
, error: onError
|
|
, type: 'GET'
|
|
});
|
|
}
|
|
|
|
var getBooks = function(q, onSuccess, onError)
|
|
{
|
|
$.ajax(
|
|
{ url: '/books' + '?q=' + encodeURIComponent(q)
|
|
, success: onSuccess
|
|
, error: onError
|
|
, type: 'GET'
|
|
});
|
|
}
|
|
```
|
|
|
|
We created a directory `static` that contains two static files: `index.html`,
|
|
which is the entrypoint to our little web application; and `ui.js`, which
|
|
contains some hand-written javascript. This javascript code assumes the two
|
|
generated functions `getPoint` and `getBooks` in scope. Therefore we need to
|
|
write the generated javascript into a file:
|
|
|
|
``` haskell
|
|
writeJSFiles :: IO ()
|
|
writeJSFiles = do
|
|
T.writeFile "static/api.js" apiJS1
|
|
jq <- T.readFile =<< Language.Javascript.JQuery.file
|
|
T.writeFile "static/jq.js" jq
|
|
```
|
|
|
|
(We're also writing the jquery library into a file, as it's also used by
|
|
`ui.js`.) `static/api.js` will be included in `index.html` and the two
|
|
generated functions will therefore be available in `ui.js`.
|
|
|
|
And we're good to go. You can start the `main` function of this file and go to
|
|
`http://localhost:8000/`. Start typing in the name of one of the authors in our
|
|
database or part of a book title, and check out how long it takes to
|
|
approximate pi using the method mentioned above.
|
|
|
|
## Customizations
|
|
|
|
Instead of calling `jquery`, you can call its variant `jqueryWith`.
|
|
Here are the type definitions
|
|
|
|
```haskell ignore
|
|
jquery :: JavaScriptGenerator
|
|
jqueryWith :: CommonGeneratorOptions -> JavaScriptGenerator
|
|
```
|
|
|
|
The `CommonGeneratorOptions` will let you define different behaviors to
|
|
change how functions are generated. Here is the definition of currently
|
|
available options:
|
|
|
|
```haskell ignore
|
|
data CommonGeneratorOptions = CommonGeneratorOptions
|
|
{
|
|
-- | function generating function names
|
|
functionNameBuilder :: FunctionName -> Text
|
|
-- | name used when a user want to send the request body (to let you redefine it)
|
|
, requestBody :: Text
|
|
-- | name of the callback parameter when the request was successful
|
|
, successCallback :: Text
|
|
-- | name of the callback parameter when the request reported an error
|
|
, errorCallback :: Text
|
|
-- | namespace on which we define the js function (empty mean local var)
|
|
, moduleName :: Text
|
|
-- | a prefix that should be prepended to the URL in the generated JS
|
|
, urlPrefix :: Text
|
|
}
|
|
```
|
|
|
|
This pattern is available with all supported backends, and default values are provided.
|
|
|
|
## Vanilla support
|
|
|
|
If you don't use JQuery for your application, you can reduce your
|
|
dependencies to simply use the `XMLHttpRequest` object from the standard API.
|
|
|
|
Use the same code as before but simply replace the previous `apiJS` with
|
|
the following one:
|
|
|
|
``` haskell
|
|
apiJS2 :: Text
|
|
apiJS2 = jsForAPI api vanillaJS
|
|
```
|
|
|
|
The rest is *completely* unchanged.
|
|
|
|
The output file is a bit different, but it has the same parameters,
|
|
|
|
``` javascript
|
|
|
|
|
|
var getPoint = function(onSuccess, onError)
|
|
{
|
|
var xhr = new XMLHttpRequest();
|
|
xhr.open('GET', '/point', true);
|
|
xhr.setRequestHeader(\"Accept\",\"application/json\");
|
|
xhr.onreadystatechange = function (e) {
|
|
if (xhr.readyState == 4) {
|
|
if (xhr.status == 204 || xhr.status == 205) {
|
|
onSuccess();
|
|
} else if (xhr.status >= 200 && xhr.status < 300) {
|
|
var value = JSON.parse(xhr.responseText);
|
|
onSuccess(value);
|
|
} else {
|
|
var value = JSON.parse(xhr.responseText);
|
|
onError(value);
|
|
}
|
|
}
|
|
}
|
|
xhr.send(null);
|
|
}
|
|
|
|
var getBooks = function(q, onSuccess, onError)
|
|
{
|
|
var xhr = new XMLHttpRequest();
|
|
xhr.open('GET', '/books' + '?q=' + encodeURIComponent(q), true);
|
|
xhr.setRequestHeader(\"Accept\",\"application/json\");
|
|
xhr.onreadystatechange = function (e) {
|
|
if (xhr.readyState == 4) {
|
|
if (xhr.status == 204 || xhr.status == 205) {
|
|
onSuccess();
|
|
} else if (xhr.status >= 200 && xhr.status < 300) {
|
|
var value = JSON.parse(xhr.responseText);
|
|
onSuccess(value);
|
|
} else {
|
|
var value = JSON.parse(xhr.responseText);
|
|
onError(value);
|
|
}
|
|
}
|
|
}
|
|
xhr.send(null);
|
|
}
|
|
|
|
|
|
```
|
|
|
|
And that's all, your web service can of course be accessible from those
|
|
two clients at the same time!
|
|
|
|
## Axios support
|
|
|
|
### Simple usage
|
|
|
|
If you use Axios library for your application, we support that too!
|
|
|
|
Use the same code as before but simply replace the previous `apiJS` with
|
|
the following one:
|
|
|
|
``` haskell
|
|
apiJS3 :: Text
|
|
apiJS3 = jsForAPI api $ axios defAxiosOptions
|
|
```
|
|
|
|
The rest is *completely* unchanged.
|
|
|
|
The output file is a bit different,
|
|
|
|
``` javascript
|
|
|
|
|
|
var getPoint = function()
|
|
{
|
|
return axios({ url: '/point'
|
|
, method: 'get'
|
|
});
|
|
}
|
|
|
|
|
|
|
|
var getBooks = function(q)
|
|
{
|
|
return axios({ url: '/books' + '?q=' + encodeURIComponent(q)
|
|
, method: 'get'
|
|
});
|
|
}
|
|
|
|
```
|
|
|
|
**Caution:** In order to support the promise style of the API, there are no onSuccess
|
|
nor onError callback functions.
|
|
|
|
### Defining Axios configuration
|
|
|
|
Axios lets you define a 'configuration' to determine the behavior of the
|
|
program when the AJAX request is sent.
|
|
|
|
We mapped this into a configuration
|
|
|
|
``` haskell
|
|
data AxiosOptions = AxiosOptions
|
|
{ -- | indicates whether or not cross-site Access-Control requests
|
|
-- should be made using credentials
|
|
withCredentials :: !Bool
|
|
-- | the name of the cookie to use as a value for xsrf token
|
|
, xsrfCookieName :: !(Maybe Text)
|
|
-- | the name of the header to use as a value for xsrf token
|
|
, xsrfHeaderName :: !(Maybe Text)
|
|
}
|
|
```
|
|
|
|
## Angular support
|
|
|
|
### Simple usage
|
|
|
|
You can apply the same procedure as with `vanillaJS` and `jquery`, and
|
|
generate top level functions.
|
|
|
|
The difference is that `angular` Generator always takes an argument.
|
|
|
|
``` haskell
|
|
apiJS4 :: Text
|
|
apiJS4 = jsForAPI api $ angular defAngularOptions
|
|
```
|
|
|
|
The generated code will be a bit different than previous generators. An extra
|
|
argument `$http` will be added to let Angular magical Dependency Injector
|
|
operate.
|
|
|
|
**Caution:** In order to support the promise style of the API, there are no onSuccess
|
|
nor onError callback functions.
|
|
|
|
``` javascript
|
|
|
|
|
|
var getPoint = function($http)
|
|
{
|
|
return $http(
|
|
{ url: '/point'
|
|
, method: 'GET'
|
|
});
|
|
}
|
|
|
|
|
|
|
|
var getBooks = function($http, q)
|
|
{
|
|
return $http(
|
|
{ url: '/books' + '?q=' + encodeURIComponent(q)
|
|
, method: 'GET'
|
|
});
|
|
}
|
|
|
|
```
|
|
|
|
You can then build your controllers easily
|
|
|
|
``` javascript
|
|
|
|
app.controller("MyController", function($http) {
|
|
this.getPoint = getPoint($http)
|
|
.success(/* Do something */)
|
|
.error(/* Report error */);
|
|
|
|
this.getPoint = getBooks($http, q)
|
|
.success(/* Do something */)
|
|
.error(/* Report error */);
|
|
});
|
|
```
|
|
|
|
### Service generator
|
|
|
|
You can also generate automatically a service to wrap the whole API as
|
|
a single Angular service:
|
|
|
|
``` javascript
|
|
app.service('MyService', function($http) {
|
|
return ({
|
|
postCounter: function()
|
|
{
|
|
return $http(
|
|
{ url: '/counter'
|
|
, method: 'POST'
|
|
});
|
|
},
|
|
getCounter: function()
|
|
{
|
|
return $http(
|
|
{ url: '/books' + '?q=' + encodeURIComponent(q), true);
|
|
, method: 'GET'
|
|
});
|
|
}
|
|
});
|
|
});
|
|
```
|
|
|
|
To do so, you just have to use an alternate generator.
|
|
|
|
``` haskell
|
|
apiJS5 :: Text
|
|
apiJS5 = jsForAPI api $ angularService defAngularOptions
|
|
```
|
|
|
|
Again, it is possible to customize some portions with the options.
|
|
|
|
``` haskell
|
|
data AngularOptions = AngularOptions
|
|
{ -- | When generating code with wrapInService, name of the service to generate, default is 'app'
|
|
serviceName :: Text
|
|
, -- | beginning of the service definition
|
|
prologue :: Text -> Text -> Text
|
|
, -- | end of the service definition
|
|
epilogue :: Text
|
|
}
|
|
```
|
|
|
|
# Custom function name builder
|
|
|
|
Servant comes with three name builders included:
|
|
|
|
- camelCase (the default)
|
|
- concatCase
|
|
- snakeCase
|
|
|
|
Keeping the JQuery as an example, let's see the impact:
|
|
|
|
``` haskell
|
|
apiJS6 :: Text
|
|
apiJS6 = jsForAPI api $ jqueryWith defCommonGeneratorOptions { functionNameBuilder= snakeCase }
|
|
```
|
|
|
|
This `Text` contains 2 Javascript functions:
|
|
|
|
``` javascript
|
|
|
|
|
|
var get_point = function(onSuccess, onError)
|
|
{
|
|
$.ajax(
|
|
{ url: '/point'
|
|
, success: onSuccess
|
|
, error: onError
|
|
, type: 'GET'
|
|
});
|
|
}
|
|
|
|
var get_books = function(q, onSuccess, onError)
|
|
{
|
|
$.ajax(
|
|
{ url: '/books' + '?q=' + encodeURIComponent(q)
|
|
, success: onSuccess
|
|
, error: onError
|
|
, type: 'GET'
|
|
});
|
|
}
|
|
|
|
```
|
|
|