Let vs. Where

From HaskellWiki

This seems to be only a matter of taste in the sense of "Declaration vs. expression_style", however there is more about it.

that is, it can be written whereever expressions are allowed.

like the pattern matching line of a function definition.

1 Advantages of let

Consider you have the function

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

However, transforming to

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

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

2 Advantages of where

Because "where" blocks are bound to a syntactic construct, they can be used to share bindings between parts of a function that are not syntactically expressions. For example:

f x
  | cond1 x   = a
  | cond2 x   = g a
  | otherwise = f (h x a)
  where
    a = w x

in order to represent the guards:

f x =
   let a = w x
   in  select (f (h x a))
          [(cond1 x, a),
           (cond2 x, g a)]

Without such a function it looks worse. You would lose the guard structure, and the heavier lexemes arguably make the resulting function harder to read:

f x
  = let a = w x
    in if cond1 x
       then a
       else if cond2 x
       then g a
       else f (h x a)

3 Lambda Lifting

One other approach to consider is that let or where can often be implemented using lambda lifting and let floating, incurring at least the cost of introducing a new name. The above example:

f x
  | cond1 x   = a
  | cond2 x   = g a
  | otherwise = f (h x a)
  where
    a = w x

could be implemented as:

f x = f' (w x) x
 
f' a x
  | cond1 x   = a
  | cond2 x   = g a
  | otherwise = f (h x a)

The auxilliary definition can either be a top-level binding, or included in f using let or where.

Categories: Style | Syntax