Re: [isabelle] argument order of List.foldr



On Sat, Mar 6, 2010 at 10:36 AM, Lucas Dixon <ldixon at inf.ed.ac.uk> wrote:
> I really like Isabelle's argument-order for fold; in particular I
> frequently fold over list in order to update some value:
>
> a |> fold f l
>  |> fold f2 l2
...
> This argument order is also nice for combine folds...
>
> fold o fold : ('a -> 'b -> 'b) -> 'a list list -> 'b -> 'b

On Sat, Mar 6, 2010 at 12:28 PM, Alexander Krauss <krauss at in.tum.de> wrote:
> it is not so much folding a binary operation like "+" but more an iteration
> of a transformation of a state. But it is the only version that composes
> (fold o fold).

These are good points. I guess both argument orders have their uses,
in different situations. I suppose I was thinking with a Haskell bias,
since partially-applied foldr is used a lot in Haskell. Also, the
"fold o fold" situation never seems to come up in Haskell either,
since for state transformers people generally use mapM with a state
monad rather than a fold.

- Brian





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