Laziness is not always good

From HaskellWiki

Generally, since Haskell is a non-strict language, one 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:

mempty = ()
mappend _ _ = ()

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

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

Is this academic nitpicking or practically relevant?

that monoid laws can be applied in every case.

You might even create an optimizer rule doing this.

and this gets lost when the optimization is applied.

The solution to this to define

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

and write

mappend (force a) (force b)

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

See also

• Haskell-Cafe on Laws and partial values
• Maintaining laziness

Categories: Idioms | Style