Re: [Haskell-cafe] coding standard question

2009/6/22 Malcolm Wallace :

Erik de Castro Lopo wrote:

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Vasili I. Galchin wrote:

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 "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.

Particularly in larger examples, it can be remarkably easy to miss an occurrence of the variable whose name you are refactoring.  The key point is that in this situation, unlike most refactorings, the compiler _cannot_ help you find the mistake with useful error messages or warnings.

What about an editor (or emacs mode or…) that support variable renaming? Maybe there is already one? Loup