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Is there any function in F# standard library that allows transform a list into a value not by per element basis?
f#functional

There's fold function, but it takes exactly one element from the list on each iteration.

I'd like to have standard function with signature

transformer:('b list -> 'a * 'b list) -> list:'b list -> 'a list

Here transformer is a function, that takes list, translates some part of it into 'a and returns 'a and a rest of list.

In general, I need a functional analog of for loop.

e.g. for_m: in: 'input -> init:'state -> transform: ('input -> 'state -> 'state * 'input) -> break: ('input -> bool) -> 'state

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let loop ``in`` init transform ``break`` =

    let rec loop_tail result rest =

        if ``break`` rest then

            result

        else

            let result, rest = transform rest result

            loop_tail result rest

    

    loop_tail init ``in``
.
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let transform stateF initialState f data =
  let rec transformAux state data acc = 
    match data with
    | hd :: tl ->
        let transformedValue = f state hd
        let transformedState = stateF state transformedValue
        transformAux transformedState tl (transformedValue :: acc)
    | [] -> List.rev acc
  transformAux initialState data []

transform (fun state x -> state + x) 1 (fun state x -> x * state) [1;2;3;4;5;6]

just in case... it is not a standard function though

By on 8/24/2009 8:10 AM ()

Sounds like Seq.unfold may also be a relevant function to look at.

-Keith

By on 8/23/2009 12:28 PM ()

You might want to check out Seq.pairwise and Seq.windowed

[link:research.microsoft.com]

though I don't think either does exactly what you describe here.

You might also check out

[link:cs.hubfs.net]

and linked threads.

By on 8/23/2009 9:32 AM ()

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