[Haskell-cafe] Re: coding standard question

Malcolm Wallace wrote:

Johan Tibell wrote:

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

The proper way to avoid shadowing in this simple case would be to make g global (and don't export it). f a b = g (a+b) (b-a) g a c = a*c

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

Clearly I simplified the example too far. Try this, only slightly more complex, example instead. Remember, the larger the example, the more likely you are to miss an occurrence.

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

Perhaps I should advocate for a new warning in GHC to cover this case: -fwarn-mixed-scopes, which could flag the use of the unprimed a, due to being bound at an outer scope.

The main reason for let and where are such "mixed scopes". (And maybe you should get a warning if the scope is not mixed, as in the initial example.) In any case I think, it does make sense to avoid name shadowing in the same way as it make sense to avoid shadowing of imported (or declared) global names by local variables (i.e. by using "pi" or "id" as variables). Surely, it may be convenient to reuse names in local scopes, but it is as hard to read such code as it is to replace shadowed variables consistently by hand. (A refactoring tool could rename shadowed variables as reliable as the compiler treats them.) Surely there's no help against mixing up names with the same type. Cheers Christian