[Haskell-beginners] Where/Let clauses and subexpression repetition/memoization

Dear Haskell Community, I have been bedeviled by a persistent confusion about one aspect of where/let clauses, which I think I can illustrate more easily with code than with words. Given code like this: averageR pxls = map (pcast . setR ravg) pxls' where (ravg,_,_) = averages pxls' pxls' = map pcast pxls What happens if I substitute pxls' for its right-hand side, averageR pxls = map (pcast . setR ravg) (map pcast pxls) where (ravg,_,_) = averages (map pcast pxls) My "superstition" here has been that the former only evaluates pxls' once, whereas the latter computes it twice. This seems like a basic issue which must have been confirmed or debunked somewhere in my readings, but it hasn't sunk in with me.