[Haskell-cafe] Re: [Haskell] View patterns in GHC: Request for
stefanor at cox.net
Wed Jul 25 16:07:26 EDT 2007
On Wed, Jul 25, 2007 at 09:35:32PM +0200, apfelmus wrote:
> => (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
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!
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