Re: [Haskell-cafe] coding standard question

On Mon, Jun 22, 2009 at 12:06 PM, Malcolm Wallace < Malcolm.Wallace@cs.york.ac.uk> wrote:

Erik de Castro Lopo > wrote:

...

Vasili I. Galchin wrote:

...

"where/let" functions use the same name for function parameters as the outer function and hence there is a "shadow" warning from the compiler.

In Haskell there is an easy way around this. Variables can be name a, a', a'' and so on. ... ... its a good idea to fix these warnings.

I would _strongly_ advise not to do that. By trying to silence the spurious warning about shadowing, there is enormous potential to introduce new bugs that were not there before.

Example:

f a b = g (a+b) (b-a) where g a c = a*c

ghc warns that g's parameter a shadows the parameter to f. So we introduce a primed identifier to eliminate the warning:

f a b = g (a+b) (b-a) where g a' c = a*c

Now, no warnings! But, oops, this function does not do the same thing. We forgot to add a prime to all occurrences of a on the right-hand-side.

Actually there's a warning: ghci> let f a b = g (a+b) (b-a) where g a' c = a*c <interactive>:1:34: Warning: Defined but not used: `a'' Cheers, Johan