Louis Wasserman wrote:
I'd like to see something resembling as-patterns in type signatures. Specifically, there are cases where I'm inclined to use (m ~ pat) in a type context when m isn't otherwise constrained, just so I can use m as an abbreviation for pat. To reduce these cases, I'd like to see the syntax m@pat allowed for use in type signatures, with m becoming simply an alias for pat. Thoughts?
I've added a ticket here http://hackage.haskell.org/trac/ghc/ticket/3545.
+1. I've often wanted this when doing type-level programming. Another nice thing this suggests is the ability to use underscore as a pattern for when you know the compiler will infer the type but it's too complex to want to write out (e.g. while experimenting). With x@_ you could even share the complex type in multiple places, since x will be bound to some actual type rather than being universally quantified. (Of course, ISTR someone's suggested the underscores in the past and had it rejected. Alas.) -- Live well, ~wren