Definition order should not be important in where block

[erikd]

Two almost identical functions below. The first parses successfully, the second fails.

compileSuccess :: Int -> Int
compileSuccess x =
	x + y
  where
    -- This works becuase 'z' is defined before it is used.
    z = 4
    y = 3 + z

compileFail :: Int -> Int
compileFail x =
	x + y
  where
    -- Using 'z' before it is defined fails.
    y = 3 + z
    z = 4

[benl]

This is only true if the expressions that are out of order don't have conflicting effects. There are also problems if one of them diverges.

For a language that uses call-by-value evaluation as default, we should probably require non-function bindings to be in the correct order. We could reasonably allow function bindings to be out of order, but I'm not sure we want to make this a special case.