Data declaration with constraint
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
I like to declare an instance for an type constructor class for the type constructor
but its methods require type constraints that depend on the particular type constructor
instance Vector T where add (Cons x) (Cons y) = Cons (x+y) -- requires Num constraint on type a
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. GHC as of version 7.2 disallows them by default (turn back on with
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
This way, functions using
T no longer need a constraint. Instead, constructing a
T a using
Cons needs a
C a context, and when something pattern matches a
Cons x, the constraint 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 classes, where
T a and
a are separate arguments.
- Henning Thielemann in Haskell-Cafe: Context for type parameters of type constructors
- Mark Nicholls in Haskell-Cafe: nice simple problem for someone struggling....