# Let vs. Where

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It is important to know that <hask>let ... in ...</hask> is an expression, |
It is important to know that <hask>let ... in ...</hask> is an expression, |
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− | that is, it can be written whereever expressions are allowed. |
+ | that is, it can be written wherever expressions are allowed. |

In contrast to that, <hask>where</hask> is bound to a surrounding syntactic construct, |
In contrast to that, <hask>where</hask> is bound to a surrounding syntactic construct, |
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like the [[pattern matching]] line of a function definition. |
like the [[pattern matching]] line of a function definition. |
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Line 113: | Line 113: | ||

</haskell> |
</haskell> |
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− | The auxilliary definition can either be a top-level binding, or included in f using <hask>let</hask> or <hask>where</hask>. |
+ | The auxiliary definition can either be a top-level binding, or included in f using <hask>let</hask> or <hask>where</hask>. |

[[Category:Style]] |
[[Category:Style]] |

## Revision as of 17:53, 4 March 2010

Haskell programmers often wonder, whether to useThis 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 thatthat is, it can be written wherever expressions are allowed.

In contrast to that,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

f x = let a = w x in case () of _ | cond1 x = a | cond2 x = g a | otherwise = f (h x a)

or a functional equivalent:

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

or a series of if-then-else expressions:

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)