Difference between revisions of "Data declaration with constraint"
(maybe multiparameter type classes are a way out) 
(Added use of GADTs to add a context to data constructors.) 

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=== Answer === 
=== Answer === 

−  +  In Haskell 98, only functions can have type constraints. 

The type constraint of a <hask>data</hask> only refers to the constructors. 
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. 
The designers of Haskell 98 do now think, that it was a bad decision to allow constraints on constructors. 

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== Solution == 
== Solution == 

+  You could use ghc's [http://www.haskell.org/ghc/docs/latest/html/users_guide/datatypeextensions.html#gadtstyle Generalised Algebraic Data Structures (GADTs)] to add an implicit context to the data constructors. 

−  But how can one bake type constraints into a type? 

⚫  
⚫  
−  For now you have to stick to [[multiparameter type class]]es, 

+  
−  where <hask>T a</hask> and <hask>a</hask> are separate arguments. 
+  A Haskell 98 workaround is to use [[multiparameter type class]]es, where <hask>T a</hask> and <hask>a</hask> are separate arguments. 
Revision as of 23:07, 4 June 2013
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
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.
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 multiparameter type classes, where T a
and a
are separate arguments.
See also
 Henning Thielemann in HaskellCafe: Context for type parameters of type constructors
 Mark Nicholls in HaskellCafe: nice simple problem for someone struggling....