View source for Data declaration with constraint

== Problem ==

=== Question ===

I have declared
<haskell>
data C a  =>  T a = Cons a
</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>.
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>.

For example: 
<haskell>
instance Vector T where
   add (Cons x) (Cons y) = Cons (x+y)    -- requires Num constraint on type a
</haskell>

=== Answer ===

In Haskell 98, only functions can have type constraints.
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. GHC as of version 7.2 disallows them by default (turn back on with <code>-XDatatypeContexts</code>).

== Solution ==

You could use ghc's [https://wiki.haskell.org/GADTs_for_dummies Generalised Algebraic Data Structures (GADTs)] to add an implicit context to the data constructors:

<haskell>
data T a where
  Cons :: C a => a -> T a
</haskell>

This way, functions using <hask>T</hask> no longer need a constraint. Instead, constructing a <hask>T a</hask> using <hask>Cons</hask> needs a <hask>C a</hask> context, and when something pattern matches a <hask>Cons x</hask>, the context comes with it.

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.

== See also ==

* Henning Thielemann in Haskell-Cafe: [http://www.haskell.org/pipermail/haskell-cafe/2004-March/005979.html Context for type parameters of type constructors]
* Mark Nicholls in Haskell-Cafe: [http://www.haskell.org/pipermail/haskell-cafe/2007-December/036762.html nice simple problem for someone struggling....]

[[Category:FAQ]]