"let" vs. "where" [was: Re: more unsafePerformIO questions (is it safe to use with ReadMode Handles)?]

Hi Andrew,

let x = expensiveComputation foo in x + x

I would certainly hope that expensiveComputation wasn't called twice, and even though the language doesn't guarantee it, I have already written code that assumed it.

I always thought that there is a tiny difference between "let" and "where": Using "let" "expensiveComputation foo" might be computed twice (depending on the compiler?). But using: x + x where x = expensiveComputation foo should compute the value for x only once. Therefore, I always try to use "where" for common subexpressions. Please correct me if I'm wrong here. Cheers, Jan