# Data declaration with constraint

## 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

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

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,
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....