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<haskell>
 
<haskell>
name :: XHtmlForm String
+
name :: Form String
 
name = input Nothing
 
name = input Nothing
 
</haskell>
 
</haskell>
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The input function takes a Maybe String, and produces a XHtmlForm String. The Maybe String is used for default values. If you give it a nothing, it won't have a default value. If you pass in a (Just "value"), it will be pre-populated with the value "value".
 
The input function takes a Maybe String, and produces a XHtmlForm String. The Maybe String is used for default values. If you give it a nothing, it won't have a default value. If you pass in a (Just "value"), it will be pre-populated with the value "value".
   
You can easily combine formlets using the combinators from Applicative. Suppose you have a User-datatype:
+
You can easily combine formlets using the Applicative Functor combinators. Suppose you have a User-datatype:
   
 
<haskell>
 
<haskell>
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userForm = User <$> name <*> inputInteger <*> input Nothing
 
userForm = User <$> name <*> inputInteger <*> input Nothing
 
</haskell>
 
</haskell>
  +
  +
You can also have more advanced widgets, like a radio-choice, that's where you use enumRadio:
  +
  +
<haskell>
  +
enumRadio :: (Monad m, Enum a) => [(a, String)] -> Maybe a -> Form a
  +
</haskell>
  +
  +
So it asks for a list of pairs with a value and the corresponding label, a possible default-value and it will return something of type a.
  +
  +
<haskell>
  +
chooseBool :: Form Bool
  +
chooseBool = enumRadio [(True, "Yes"), (False, "No")] True
  +
</haskell>
  +
  +
Now we have a widget for booleans that we can use everywhere in our forms!
   
 
== The basics ==
 
== The basics ==
  +
  +
=== Simple validation ===
  +
=== Monadic validation ===
   
 
== How it works ==
 
== How it works ==
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* [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/formlets Formlets library on hackage]
 
* [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/formlets Formlets library on hackage]
 
* [http://groups.inf.ed.ac.uk/links/formlets/ Papers on formlets]
 
* [http://groups.inf.ed.ac.uk/links/formlets/ Papers on formlets]
  +
* [http://en.wikibooks.org/wiki/Haskell/Applicative_Functors Applicative Functors wikibook]

Revision as of 09:13, 12 August 2008

Contents

1 Introduction

Formlets are a way of building HTML forms that are type-safe, handle errors, abstract and are easy to combine into bigger forms. Here's an example:

name :: Form String
name = input Nothing

The input function takes a Maybe String, and produces a XHtmlForm String. The Maybe String is used for default values. If you give it a nothing, it won't have a default value. If you pass in a (Just "value"), it will be pre-populated with the value "value".

You can easily combine formlets using the Applicative Functor combinators. Suppose you have a User-datatype:

data User = User {name :: String, age :: Integer, email :: String}

You can then build a form that produces a user:

userForm :: Form User
userForm = User <$> name <*> inputInteger <*> input Nothing

You can also have more advanced widgets, like a radio-choice, that's where you use enumRadio:

enumRadio :: (Monad m, Enum a) => [(a, String)] -> Maybe a -> Form a

So it asks for a list of pairs with a value and the corresponding label, a possible default-value and it will return something of type a.

chooseBool :: Form Bool
chooseBool = enumRadio [(True, "Yes"), (False, "No")] True

Now we have a widget for booleans that we can use everywhere in our forms!

2 The basics

2.1 Simple validation

2.2 Monadic validation

3 How it works

3.1 Advanced: rolling your own output type

4 References