# Data declaration with constraint

### From HaskellWiki

(Difference between revisions)

(constraint must be in front of the type) |
(maybe multi-parameter type classes are a way out) |
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data C a => T a = Cons a |
data C a => T a = Cons a |
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</haskell> |
</haskell> |
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− | 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>. |
+ | 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>. |
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+ | I like to declare an instance for an type constructor class for the type constructor <hask>T</hask> |
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+ | but its methods require type constraints that depend on the particular type constructor <hask>T</hask>. |
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+ | |||

+ | E.g. |
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+ | <haskell> |
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+ | instance Vector T where |
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+ | add (Cons x) (Cons y) = Cons (x+y) -- requires Num constraint on type a |
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+ | </haskell> |
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=== Answer === |
=== Answer === |
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== Solution == |
== Solution == |
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− | 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. |
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+ | |||

+ | For now you have to stick to [[multi-parameter type class]]es, |
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+ | where <hask>T a</hask> and <hask>a</hask> are separate arguments. |
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## 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....