Difference between revisions of "Data declaration with constraint"
(constraint must be in front of the type) 
Artyom Kazak (talk  contribs) 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> 

⚫  
+  
⚫  
+  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 === 
=== 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. 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> 

+  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 [[multiparameter 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
Contents
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 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....