[Haskell-cafe] Accumulator value for "and" and "or"
stefanor at cox.net
Thu Sep 20 23:02:30 EDT 2007
On Fri, Sep 21, 2007 at 03:48:25AM +0100, PR Stanley wrote:
> or = foldl (||) False
> and = foldl (&&) True
> I can understand the rationale for the accumulator value - True &&  where
>  = True and True ||  where  = False
> Other than the practical convenience is there a reason for having the empty
> list in and and or equating to True and False?
> Thanks, Paul
It makes and and or into monoid morphisms.
and (xs ++ ys) = and xs && and ys
or (xs ++ ys) = or xs || or ys
(PS are you sure you meant foldl? the standard definitons use foldr)
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