Let vs. Where
Haskell programmers often wonder, whether to use
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
However, transforming to
f :: State s a f = State $ \x -> y where y = ... x ...
will not work, because
where refers to the pattern matching
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