[Haskell-cafe] Simple list processing

[Don Stewart]

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