[Haskell-cafe] Simple list processing

[Thomas M. DuBuisson]

Why is there no mapAccumL' (strict)?  Just a library deficiency that we
can remedy or am I missing something?

Don Stewart wrote:
> andrewcoppin:
> > OK, so this is a fairly basic question about list processing.
> > 
> > Several times now, I have found myself wanting to process a list to 
> > produce a new list. However, the way later elements are processed 
> > depends on what the earlier elements are - in other words, this isn't a 
> > simple "map".
> > 
> > What is the best way to handle this?
> > 
> > According to the theory, anything that consumes a list and produces a 
> > value is some kind of fold. [Assuming it traverses the list in a 
> > sensible order!] So it looks like you could implement this as a fold. 
> > But should that be a LEFT-fold or a RIGHT-fold? (I always get confused 
> > between the two!)
> > 
> 
> Sounds like a mapAccum, a combination of map and fold,
> 
>     http://haskell.org/ghc/docs/latest/html/libraries/base/Data-List.html#v%3AmapAccumL
> 
> If you gave a concrete example it would be easier to diagnose.
> 
> -- Don
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