(if and list construction)
Revision as of 20:12, 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
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.
It is common source of too much strictness to make decisions too early and thus duplicate code in the decision branches. Intuitively spoken, the bad thing about code duplication (stylistic questions put aside) is, that the run-time system cannot see that in the branches some things are equal and do it in common before the critical decision. Actually, the compiler and run-time system could be "improved" to do so, but in order to keep things predictable, they do not do so. Even more, this behaviour is required by theory, since by pushing decisions to the inner of an expression you change the semantics of the expression. So we return to the question, what the programmer actually wants.
Now, do you think this expression
if b then [x] else y:ys
is maximally lazy? It seems so, but actually it is not. In both branches we create non-empty lists, but the run-time system cannot see this.It is
let z:zs = if b then [x] else y:ys in z:zs
This error can only catched at run-time which is bad. We can avoid it using the single constructor pair type.
let (z,zs) = if b then (x,) else (y,ys) in z:zs
which can be abbreviated to
uncurry (:) (if b then (x,) else (y,ys))
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.