# Data declaration with constraint

### From HaskellWiki

## 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....