On Wed, Jul 25, 2007 at 09:35:32PM +0200, apfelmus wrote:
Integer => (forall a . ViewInt a => a)
can even be done implicitly and for all types. Together with the magic class View, this would give real views.
Jón Fairbairn wrote:
It's essential to this idea that it doesn't involve any new pattern matching syntax; the meaning of pattern matching for overloaded functions should be just as transparent as for non-overloaded ones.
That's what the real views would do modulo the probably minor inconvenience that one would need to use (:<) and (EmptyL) instead of (:) and []. I doubt that the latter can be reused.
It's possible to go even simpler, and implement views via a simple desugaring without altering the typechecking kernel at all. (for simplicity of exposition, assume pattern matches have already been compiled to flat cases using Johnsson's algorithm; in particular the patterns mentioned consist of exactly one constructor, not zero) case scrut of pat -> a _ -> b ==> realcase (Prelude.view scrut) of pat -> a _ -> b Where in the Prelude (or the current type environment, if -fno-implicit-prelude) we have: class View a c | c -> a where view :: a -> c and we provide a deriving-form for View which generates View Foo Foo where view = id. Or, a rule which does that automatically if no explicit instance of View _ Foo is in the current module. Or, remove the fundep and add an instance View a a where view = id to the Prelude. Option 3 makes definitions more polymorphic. Options 1 and 2 keep the same level of polymorphism as before; 1 is simpler but breaks old code. Note that none of these options supports the value input feature; we need new syntax to support non-binding identifiers in patterns! Stefan