Let vs. Where

Revision as of 15:22, 14 September 2007 by Lemming (Talk | contribs)

Haskell programmers often wonder, whether to use

let

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