Partially ordered collections (revisited)

Tom Pledger Tom.Pledger at
Fri Nov 21 08:48:28 EST 2003

Graham Klyne writes:
 | Ah, yes.  I've grown a little wary of foldl, but I see it's
 | appropriate in this case.  As it's part of the standard prelude, I
 | personally have no qualms about depending upon it ... or should I?

(Hope you don't mind my returning this to haskell-cafe.)

Most of foldl's bad reputation comes from stack overflows, because it
builds deeply nested suspensions such as
    foldl f z [1..99]
    --> f (... (f (f z 1) 2) ...) 99
*regardless* of the strictness of f.

Contrast that with foldr
    foldr f z [1..99]
    --> f 1 (foldr f z [2..99])
which enters f straight away.

Your particular f, i.e. add, is strict in both its parameters, so I
don't think the choice between foldl and foldr will affect whether you
get stack overflows.  However, if you do get stack overflows, try a
strict folding function, such as a copy of Hugs's Prelude.foldl' (not

- Tom

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