[Haskell-cafe] Re: [Haskell] View patterns in GHC: Request for
drl at cs.cmu.edu
Thu Jul 26 11:28:03 EDT 2007
I think what you're describing is equivalent to making the "implicit
view function" syntax so terse that you don't write anything at all. So
the pattern 'p' is always (view -> p). This seems like a pretty
I don't think the version with the functional dependency works (unless
you adopt some form of scoped type class instances, as you suggest
below), because then if you want to use a datatype as a view, you can no
longer pattern match on the datatype itself at all! Even with some form
of scoping, you can't decompose the view datatype as itself and as a
view in the same scope.
The non-functional type class will make everything very polymorphic
(e.g., where we used to infer a type based on the datatype constructors
that occurred, we will now say that it's anything that can be viewed as
So, this syntax affects a lot of code, existing or otherwise, that
doesn't use view patterns, which is something we're trying to avoid.
On Jul25, Stefan O'Rear wrote:
> 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!
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