Holes in GHC

[Simon Peyton-Jones]

|  The primary goal is to make this part of GHCi. Say, you're working on
|  a file Foo.hs in your favorite editor, and you have: ....

Aha.  That is helpful (below).  Start a GHC wiki page to describe?

Now, if I compile

	{-# LANGUAGE ImplicitParams #-}
	module Foo where

	foo = foldr ?x 'x' [True,False]

I get this:
	Foo.hs:4:13:
	    Unbound implicit parameter (?x::Bool -> Char -> Char)
 	     arising from a use of implicit parameter `?x'

That looks pretty close to what you want, no?   You want the error 

	Foo.hs:4:13:
 	   Found a hole of type: Bool -> Char -> Char

Same information, with different wording.

This reminds me of the recently implemented -fdefer-type-errors.  You'd want to be able to *run* a program with hole in it, getting the "unfilled hole of type Bool -> Char -> Char" error if you ever need to evaluate the hole.

Anyway, now I understand more clearly what you are trying to do.  Thanks.

Simon
 
|  The primary goal is to make this part of GHCi. Say, you're working on
|  a file Foo.hs in your favorite editor, and you have:
|  
|  ---
|  
|  foo = foldr __ 0 [1..5]
|  
|  ---
|  
|  And you have no idea what you should use at the location of the "__".
|  You bring up GHCi, and load it as a module:
|  
|  $ ghci
|  GHCi, version 7.5.20120126: http://www.haskell.org/ghc/  :? for help
|  Loading package ghc-prim ... linking ... done.
|  Loading package integer-gmp ... linking ... done.
|  Loading package base ... linking ... done.
|  Prelude> :load Foo.hs
|  [1 of 1] Compiling Main             ( Foo.hs, interpreted )
|  Found a hole at Foo.hs:1:13-14: Integer -> Integer -> Integer
|  ...
|  
|  You notice it needs a function, so you make some more changes and hit
|  save, so Foo.hs now contains:
|  
|  ---
|  
|  foo = foldr (\x -> __) 0 [1..5]
|  
|  ---
|  
|  You reload GHCi, to see if you made progress:
|  
|  *Main> :r
|  [1 of 1] Compiling Main             ( Foo.hs, interpreted )
|  Found a hole at Foo.hs:1:20-21: Integer -> Integer
|  ...
|  
|  And that's it. It might help IDEs later on, but that is not our goal.
|  
|  Regards,
|  Thijs Alkemade