the code for foldl:

FoldL(s) : ([x | xs], y) -> (xs, (x, y))

foldl(s) =
rec x( ([], y) -> y + FoldL(s); x)

tranposes the arguments in the call:
(x,y) in the auxiliary rule Foldl.
This puts the latest list element x on the LHS
and the accumulator on the RHS of the operator

The problem is manifest in calls like:

<foldl(conc)> ([[1],[2],[3]],[])

which yields [3,2,1]

and

foldl(subtr) ([1,2,3],0)

which gives 2 rather than -6

One possible fix is the redefinition:

FoldL(s) : ([x | xs], y) -> (xs, (y, x))

Submitted on 10 January 2008 at 02:44