why is this legal

[David Feuer]

Hal Daume III wondered:
> f x = f' 0 x
>     where f' acc [] = acc
>           f acc (x:xs) = f' (x+acc) xs

> why are we allowed to rebind f in the where clause?  
this is clearly a
> typo (in this instance) but it seems really strange to 
me that this would
> be allowed.

Because this definition is equivalent to

f x = let f' acc [] = acc
	  f acc (x:xs) = f' (x+acc) xs
      in f' 0 x

More generally (if I am not mistaken...)

g p1 q1 ... | c1 = ...
	    | c2 = ...
            | ....
   where <bindings>

is the same as

g p1 q1 ... = let <bindings>
	      in case () of
		    _ | c1 -> ...
		      | c2 -> ...

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