Re: [Haskell-cafe] let vs. where

On Tue, Nov 13, 2007 at 11:41:20AM -0800, Justin Bailey wrote:

On Nov 13, 2007 10:56 AM, John Lato wrote:

...

I know there are several important differences between let-expressions and where-clauses regarding scoping and the restriction of "where" to a top-level definition. However, frequently I write code in which

One place I find it useful is when there is a common computed value that is used throughout a function definition. For example, imagine some function that uses the length of a list passed in:

someFunction ls a b c = ... (length ls) where someAuxFunction x y = ... length ls .. someOtherFunction x y = ... length ls ...

a where clause can capture that calculation, make sure it's only done once, and shared throughout the function definition:

someFunction ls a b c = ... listLen ... where listLen = length ls someAuxFunction x y = ... listLen ... someOtherFunction x y = ... listLen ...

Notice a let clause wouldn't do it above, because "length ls" is called inside other functions defined in the where clause. Of course everything could be moved to a "let" clause in the function body. At that point I think it's purely stylistic.

A let clause would work fine here: someFunction ls a b c = let listLen = length ls someAuxFunction x y = ... listLen ... someOtherFunction x y = ... listLen ... in ... listLen ... it's just that you don't want to mix let and where clauses, because then things get confusing. Even if it worked with both, noone would know the binding rules. -- David Roundy Department of Physics Oregon State University