Difference between revisions of "Multiple instances"

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m (type parameter order)
(Monoid)
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I like to define multiple [[type class instance]]s for the same pair of class and type.
 
I like to define multiple [[type class instance]]s for the same pair of class and type.
  +
Sometimes I also need two instances where the order of type parameters is different.
  +
E.g. I want to define two <hask>Functor</hask> instances for a pair:
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One instance where the first member is mapped and another instance where the second member is mapped.
 
How is it possible?
 
How is it possible?
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== Answer ==
 
== Answer ==
   
You can achieve it if you keep class and type definitions in other modules than the instance declarations.
+
You can achieve the first goal if you keep class and type definitions in other modules than the instance declarations.
 
These instances are then called [[orphan instance]]s.
 
These instances are then called [[orphan instance]]s.
 
However you won't be lucky with this solution, since you must ensure,
 
However you won't be lucky with this solution, since you must ensure,
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since instance declarations are automatically imported and cannot be hidden.
 
since instance declarations are automatically imported and cannot be hidden.
 
Even more also modules which import conflicting modules only indirectly conflict itself.
 
Even more also modules which import conflicting modules only indirectly conflict itself.
It is also not possible to define instances with respect to different type parameter orders,
 
  +
The second problem cannot be solved with this approach, too.
as it is required for type constructor classes like <hask>Functor</hask>, <hask>Monad</hask>, and others.
 
   
 
Thus multiple instances should be avoided, and a safe way to do this is to avoid orphan instances.
 
Thus multiple instances should be avoided, and a safe way to do this is to avoid orphan instances.
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If you do not fear [[Use of language extensions|language extensions]] you can simplify this task considerably using the <code>GeneralizedNewtypeDeriving</code> feature.
 
If you do not fear [[Use of language extensions|language extensions]] you can simplify this task considerably using the <code>GeneralizedNewtypeDeriving</code> feature.
 
The custom instance can be defined for the class/newtype pair and it is not orphan, if it is defined where newtype is introduced.
 
The custom instance can be defined for the class/newtype pair and it is not orphan, if it is defined where newtype is introduced.
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Using <hask>newtype</hask> you can also change the order of type parameters or give type parameters a fixed type.
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Example:
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There are so many types and operations that exhibit a monoid structure,
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but it would not be useful to call all the operations <hask>mempty</hask> and <hask>mappend</hask>.
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It is however useful to call them via the Monoid interface sometimes, e.g. in the <hask>Writer</hask> monad.
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Thus the module <hask>Data.Monoid</hask> provides several newtype wrappers for common monoids.
   
   

Revision as of 15:43, 20 January 2009

Question

I like to define multiple type class instances for the same pair of class and type. Sometimes I also need two instances where the order of type parameters is different. E.g. I want to define two Functor instances for a pair: One instance where the first member is mapped and another instance where the second member is mapped. How is it possible?


Answer

You can achieve the first goal if you keep class and type definitions in other modules than the instance declarations. These instances are then called orphan instances. However you won't be lucky with this solution, since you must ensure, that two modules with conflicting instances declarations are never imported together, since instance declarations are automatically imported and cannot be hidden. Even more also modules which import conflicting modules only indirectly conflict itself. The second problem cannot be solved with this approach, too.

Thus multiple instances should be avoided, and a safe way to do this is to avoid orphan instances. You can achieve this by wrapping the type in a newtype and lift all required instances to that new type. If you do not fear language extensions you can simplify this task considerably using the GeneralizedNewtypeDeriving feature. The custom instance can be defined for the class/newtype pair and it is not orphan, if it is defined where newtype is introduced. Using newtype you can also change the order of type parameters or give type parameters a fixed type.

Example: There are so many types and operations that exhibit a monoid structure, but it would not be useful to call all the operations mempty and mappend. It is however useful to call them via the Monoid interface sometimes, e.g. in the Writer monad. Thus the module Data.Monoid provides several newtype wrappers for common monoids.


See also