(Wadler's force function, if-then-else)
Revision as of 17:59, 28 December 2008
One of Haskell's main features is non-strict semantics, which in is implemented by lazy evaluation in all popular Haskell compilers. However many Haskell libraries found on Hackage are implemented just as if Haskell would be a strict language. This leads to unnecessary inefficiencies, memory leaks and, we suspect, unintended semantics. In this article we want to go through some techniques on how to check lazy behaviour on functions, examples of typical constructs which break laziness without need, and finally we want to link to techniques that may yield the same effect without laziness.
1 Checking laziness
2 Laziness breakers
2.1 Maybe, Either, Exceptions
Wadler's force function
The following looks cumbersome:
let (Just x) = y in Just x
parsers - leave Maybe where no Maybe is required
2.2 Early decision
Another source of too much strictness is Be aware that the following two expression are not equivalent.
-- less lazy if b then f x else f y -- more lazy f (if b then x else y)
which is a difference in non-strict semantics.Consider e.g.
pattern match on (,) is better than pattern match on (:), because the first one has no alternative constructor
laziness encoded in uncurry
if then else
2.3 Strict pattern matching in a recursionThe implementation of the
What happened? The reason was a too strict pattern matching.
Consider the following correct implementation:
partition :: (a -> Bool) -> [a] -> ([a], [a]) partition p = foldr (\x ~(y,z) -> if p x then (x : y, z) else (y, x : z)) (,)
2.4 List reversalAny use of the list function
since when you access the first element of a reversed list, then all nodes of the input list must be evaluated and stored in memory. Think twice whether it is really needed. The article Infinity and efficiency shows how to avoid list reversal.
From the above issues you see that it laziness is a fragile thing. Only one moment where you do not pay attention and a function, carefully developed with laziness in mind, is no longer lazy, when you call it. The type system can almost not help you hunting laziness breakers and there is little support by debuggers. Thus detection of laziness breakers, often requires understanding of a large portion of code, which is against the idea of modularity. Maybe for your case you might prefer a different idiom, that achieves the same goals in a safer way. See e.g. the Enumerator and iteratee pattern.