# Difference between revisions of "Data declaration with constraint"

(maybe multi-parameter type classes are a way out) |
(Added use of GADTs to add a context to data constructors.) |
||

Line 20: | Line 20: | ||

=== 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 26: | 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. | |

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

+ | 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

## Problem

### Question

I have declared

```
data C a => T a = Cons a
```

and I hoped that now the type checker knows,
that every value of type `T a`

satisfies the type constraint on `a`

.
I like to declare an instance for an type constructor class for the type constructor `T`

but its methods require type constraints that depend on the particular type constructor `T`

.

E.g.

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

### Answer

In Haskell 98, only functions can have type constraints.
The type constraint of a `data`

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

## 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, where `T a`

and `a`

are separate arguments.

## 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....