Difference between revisions of "Data declaration with constraint"

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(short introduction with reference to current discussion)
 
(Added use of GADTs to add a context to data constructors.)
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I have declared
  
I have declared
 
<haskell>
  
<haskell>
−
data T a = C a => Cons a
 +
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>.
  +
  +
E.g.
  +
<haskell>
  +
instance Vector T where
  +
add (Cons x) (Cons y) = Cons (x+y) -- requires Num constraint on type a
 
</haskell>
  
</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>.
  
   
 
=== Answer ===
  
=== Answer ===
   
−
Only functions can have type constraints.
 +
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.
Line 17: Line 26:
 
== Solution ==
  
== Solution ==
   
  +
You could use ghc's [http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#gadt-style Generalised Algebraic Data Structures (GADTs)] to add an implicit context to the data constructors.
−
But how can one bake type constraints into a type ? ...
  
  +
  +
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.
   
   

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 multi-parameter type classes, where T a and a are separate arguments.


See also
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