Allow more/generalize let/where pattern bindings.

[bergmark]

As reported in #310 these are examples of unsupported patterns:

where
  serverDate = year ++ "-" ++ month ++ "-" ++ day
  (month:day:year:[]) = split jqDate '/'

where
  pos :: (Int, Int, Int)
  pos@(x, y, z) = (1, 2, 3) -- This is the culprit.

... and more. The code is in Fay.Compiler.Decl.compilePatBind, the general case is pretty hacky and can be improved. Note that most if not all patterns are implemented for case statements already.

[osa1]

@bergmark what's the status of this? I guess I could work on this.

I think in Haskell every pattern matching construct gets desugared into case expressions with one level of pattern matching. I think we can use similar thing and remove all pattern matching constructs except case expressions in desugaring phase.

[bergmark]

@osa1 that sounds great!

No one is working on it so please go ahead :)

[bergmark]

@osa1 Did you start working on this? I think it would simplify my typeclass branch a lot, so please let me know if you are working on it or if I should tackle it myself.

I would like every top level declaration to desugar into a single unguarded case:

f 1 = x
f y | p = z
=>
f w = case w of
  1 -> x
  y | p -> z

[osa1]

Ahh, sorry @bergmark couldn't spare my time to work on this.

The algorithm should not be very hard and IIRC it's should be explained in The Implementation of Functional Programming Languages chapter 5.

[bergmark]

Alright I'll take a look at that, thanks!