# Data declaration with constraint

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Revision as of 23:26, 19 September 2016 by Artyom Kazak (Talk | contribs)

## Contents |

## 1 Problem

### 1.1 Question

I have declared

data C a => T a = Cons a

T a

a

T

T

For example:

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. GHC as of version 7.2 disallows them by default (turn back on with `-XDatatypeContexts`

).

## 2 Solution

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

data T a where Cons :: C a => a -> T a

T

T a

Cons

C a

Cons x

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....