Difference between revisions of "Data declaration with constraint"

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(constraint must be in front of the type)
m (Solution: Change the link again)
 
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data C a => T a = Cons a
 
data C a => T a = Cons 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>.
 
  +
 
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
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add (Cons x) (Cons y) = Cons (x+y) -- requires Num constraint on type a
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</haskell>
   
 
=== Answer ===
 
=== Answer ===
   
Only functions can have type constraints.
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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.
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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 ==
 
== Solution ==
   
But how can one bake type constraints into a type ? ...
 
  +
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>
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data T a where
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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 ==
 
== See also ==

Latest revision as of 23:26, 19 September 2016

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.

For example:

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. GHC as of version 7.2 disallows them by default (turn back on with -XDatatypeContexts).

Solution

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 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 classes, where T a and a are separate arguments.

See also