# Let vs. Where

(Difference between revisions)

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.

## 1 Advantages of let

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
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```

## 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 expression style, you might instead use a functional equivalent of
case

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.