# Difference between revisions of "Laziness is not always good"

(space leak and selectors) |
m (Removed extra line-breaks.) |
||

Line 1: | Line 1: | ||

− | Generally, since Haskell is a [[Non-strict_semantics|non-strict]] language, |
+ | Generally, since Haskell is a [[Non-strict_semantics|non-strict]] language, you should try to make a function [[maintaining laziness|least strict]]. |

− | you should try to make a function [[maintaining laziness|least strict]]. |
||

This is in many cases the best semantics and the most efficient implementation. |
This is in many cases the best semantics and the most efficient implementation. |
||

However, here is an important exception from the rule: |
However, here is an important exception from the rule: |
||

Line 14: | Line 14: | ||

forall a. mappend a mempty = a |
forall a. mappend a mempty = a |
||

</haskell> |
</haskell> |
||

− | You find that it is not <hask>mappend mempty undefined = undefined</hask>, |
+ | You find that it is not <hask>mappend mempty undefined = undefined</hask>, but <hask>mappend mempty undefined = mempty</hask>. |

− | but <hask>mappend mempty undefined = mempty</hask>. |
||

Is this academic nitpicking or practically relevant? |
Is this academic nitpicking or practically relevant? |
||

− | I think it is the latter one, because a <hask>Monoid</hask> instance implicitly promises |
+ | I think it is the latter one, because a <hask>Monoid</hask> instance implicitly promises that monoid laws can be applied in every case. |

− | that monoid laws can be applied in every case. |
||

A programmer expects that every occurence of <hask>mappend mempty a</hask> can be safely replaced by <hask>a</hask>. |
A programmer expects that every occurence of <hask>mappend mempty a</hask> can be safely replaced by <hask>a</hask>. |
||

You might even create an [[Playing by the rules|optimizer rule]] doing this. |
You might even create an [[Playing by the rules|optimizer rule]] doing this. |
||

− | The above implementation of <hask>mappend</hask> however evaluates its operands lazily, |
+ | The above implementation of <hask>mappend</hask> however evaluates its operands lazily, and this gets lost when the optimization is applied. |

− | and this gets lost when the optimization is applied. |
||

The solution of this issue is to define |
The solution of this issue is to define |

## Latest revision as of 16:21, 21 September 2011

Generally, since Haskell is a non-strict language, you should try to make a function least strict.
This is in many cases the best semantics and the most efficient implementation.
However, here is an important exception from the rule:
Consider the `Monoid`

instance of the null type `()`

:

```
mempty = ()
mappend _ _ = ()
```

These functions are least strict, but have a subtle problem:
They do not generally satisfy the monoid laws.
Remind you: `mempty`

must be the identity element with respect to `mappend`

:

```
forall a. mappend mempty a = a
forall a. mappend a mempty = a
```

You find that it is not `mappend mempty undefined = undefined`

, but `mappend mempty undefined = mempty`

.
Is this academic nitpicking or practically relevant?
I think it is the latter one, because a `Monoid`

instance implicitly promises that monoid laws can be applied in every case.
A programmer expects that every occurence of `mappend mempty a`

can be safely replaced by `a`

.
You might even create an optimizer rule doing this.
The above implementation of `mappend`

however evaluates its operands lazily, and this gets lost when the optimization is applied.

The solution of this issue is to define

```
mempty = ()
mappend () () = ()
force :: () -> ()
force _ = ()
```

and write

```
mappend (force a) (force b)
```

instead of `mappend a b`

.

If you find that example too academic, you can choose any other data type with one constructor instead.

## Exercise

Find out whether it would help to define `mempty = undefined`

.

## See also

- Haskell-Cafe on Laws and partial values
- Haskell-Cafe on a space leak caused by the garbage collector that did not recognize a selector-like function call
- Maintaining laziness