Let vs. Where

Haskell programmers often wonder, whether to use let or where. This seems to be only a matter of taste in the sense of "Declaration vs. expression_style", however there is more about it.

It is important to know that let ... in ... is an expression, that is, it can be written whereever expressions are allowed. In contrast to that, where is bound to a surrounding syntactic construct, like the pattern matching line of a function definition.

Consider you have the function

f :: s -> (a,s)
f x = y
   where y = ... x ...

and later you decide to put this into the Control.Monad.State monad. However, transforming to

f :: State s a
f = State $ \x -> y
   where y = ... x ...

will not work, because where refers to the pattern matching f =, where no x is in scope.

In contrast, if you had started with let, then you wouldn't have trouble.

f :: s -> (a,s)
f x =
   let y = ... x ...
   in  y

This is easily transformed to:

f :: State s a
f = State $ \x ->
   let y = ... x ...
   in  y