# Data declaration with constraint

### From HaskellWiki

(Difference between revisions)

(short introduction with reference to current discussion) |
(maybe multi-parameter type classes are a way out) |
||

(One intermediate revision by one user not shown) | |||

Line 5: | Line 5: | ||

I have declared |
I have declared |
||

<haskell> |
<haskell> |
||

− | data T a = C a => Cons a |
+ | data C a => T a = Cons a |

+ | </haskell> |
||

+ | and I hoped that now the type checker knows, |
||

+ | that every value of type <hask>T a</hask> satisfies the type constraint on <hask>a</hask>. |
||

+ | I like to declare an instance for an type constructor class for the type constructor <hask>T</hask> |
||

+ | but its methods require type constraints that depend on the particular type constructor <hask>T</hask>. |
||

+ | |||

+ | E.g. |
||

+ | <haskell> |
||

+ | instance Vector T where |
||

+ | add (Cons x) (Cons y) = Cons (x+y) -- requires Num constraint on type a |
||

</haskell> |
</haskell> |
||

− | and I hoped that now the type checker knows, that every value of type <hask>T a</hask> satisfies the type constraint on <hask>a</hask>. |
||

=== Answer === |
=== Answer === |
||

Line 17: | Line 16: | ||

== Solution == |
== Solution == |
||

− | But how can one bake type constraints into a type ? ... |
+ | But how can one bake type constraints into a type? |

+ | You cannot. One should insert a discussion here, whether it is sensible to want this. |
||

+ | |||

+ | For now you have to stick to [[multi-parameter type class]]es, |
||

+ | where <hask>T a</hask> and <hask>a</hask> are separate arguments. |
||

## Revision as of 17:43, 21 December 2007

## Contents |

## 1 Problem

### 1.1 Question

I have declared

data C a => T a = Cons a

and I hoped that now the type checker knows,

that every value of typeT a

a

T

T

E.g.

instance Vector T where add (Cons x) (Cons y) = Cons (x+y) -- requires Num constraint on type a

### 1.2 Answer

Only functions can have type constraints.

The type constraint of adata

The designers of Haskell 98 do now think, that it was a bad decision to allow constraints on constructors.

## 2 Solution

But how can one bake type constraints into a type? You cannot. One should insert a discussion here, whether it is sensible to want this.

For now you have to stick to multi-parameter type classes,

whereT a

a

## 3 See also

- Henning Thielemann in Haskell-Cafe: Context for type parameters of type constructors
- Mark Nicholls in Haskell-Cafe: nice simple problem for someone struggling....