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type constraints on sealed types
f#compiler

I'm writing a generic class that allows for a vastly more efficient implementation for certain types (basically the integer/char/byte types). IIRC C# allows you to define multiple specializations like

class GenericClass<t> : FastGenericClassImpl_Int16 where t: Int16 {...}

class GenericClass<t> : FastGenericClassImpl_Int32 where t: Int32 {...}

etc. When I try this in F# the compiler throws errors like "Invalid constraint: the type used for the constraint is sealed, which means the constraint could only be satisfied by at most one solution" and "Duplicate definition of type, exception or module".

Is there some trick to pulling it off in F#? It's clunky to implement but I could have an F# project with the implementing classes, and a C# project that exposes the various GenericClass<t> specializations as above. It's clunky to implement but the user is mostly shielded from this (other than having to reference two assemblies for an apparently simple class).

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C# won't allow this either (for the same reasons: you can't have multiple definitions of GenericClass<t>, and types used for constraints can't be sealed). However, it looks to me like perhaps what you really want is for the inheritance to go the other direction:

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type GenericClass<'a> = ...
type FastGenericClassImpl_Int16 =
  inherit GenericClass<int16>
  ...
type FastGenericClassImpl_Int32 =
  inherit GenericClass<int32>
  ...
By on 7/31/2010 6:06 AM ()

You're right. I dug out some of my old C# code and that's what I did. The way I hid it from the users was to hide the constructor and make them use a static Create() method on the generic class that returned the appropriate concrete implementation for the type they wanted.

By on 7/31/2010 11:26 AM ()

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