# Data declaration with constraint

### From HaskellWiki

(Difference between revisions)

(constraint must be in front of the type) |
(Added use of GADTs to add a context to data constructors.) |
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data C a => T a = Cons a | data C a => T a = Cons 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>. | + | 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> | ||

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

− | + | In Haskell 98, only functions can have type constraints. | |

The type constraint of a <hask>data</hask> only refers to the constructors. | The type constraint of a <hask>data</hask> only refers to the constructors. | ||

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

Line 17: | Line 26: | ||

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

− | + | You could use ghc's [http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#gadt-style Generalised Algebraic Data Structures (GADTs)] to add an implicit context to the data constructors. | |

+ | |||

+ | There has been some discussion about whether it is sensible to want this. | ||

+ | |||

+ | A Haskell 98 workaround is to use [[multi-parameter type class]]es, where <hask>T a</hask> and <hask>a</hask> are separate arguments. | ||

## Revision as of 23:07, 4 June 2013

## 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

In Haskell 98, 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

You could use ghc's Generalised Algebraic Data Structures (GADTs) to add an implicit context to the data constructors.

There has been some discussion about whether it is sensible to want this.

A Haskell 98 workaround is to use 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....