"for-with" syntactic sugar for folds

[reinux]

I propose we add syntax sugar to turn

let result =
  for s with t in ts do
    s + t

into

let result =
  ts
  |> #Collection.fold (fun s t -> s + t) s

The with keyword introduces the initial state, and the value inside the for expression is expected to be the same as the initial state.

The existing ways of approaching this problem in F# are

fold

Folds are widely used yet slightly unwieldy, especially for beginners but even for experienced users. Nested folds tend to get really messy if we aren't extra careful with formatting. There are also several ways to write them: (state, list) ||> fold f, list |> fold f state, fold f state list. Each style has its detractors, often not without good reason. Only the first offers type inference for both state and `list within the function, but is a somewhat obscure approach.

let rec f s ts = ...

Ad-hoc recursive functions are a bit verbose, so we don't use them unless we really need to.

let mutable s in for t in ts do s <- ...

Finally, there probably isn't anything wrong with using a mutable accumulator, but it's just not something people like to reach for because it feels icky.

Pros and Cons

The advantages of making this adjustment to F# are

• Conducive to useful type inference (because s is specified prior to the function)
• Much easier for new FP users to understand without sacrificing immutability
• Better legibility for experienced users in many situations, e.g. when nested
• Alleviate the need for closing brackets at the end of the fold function or for trailing arguments, which tend to be a bit unsightly following long funs
• Natural addition to the for loop

The disadvantages of making this adjustment to F# are

Yet more syntax to document and learn. On the other hand, it seems quite intuitive and hence easy to remember.

I don't think it should require any changes to the API for CEs, but I could be wrong.

Extra information

Estimated cost (XS, S, M, L, XL, XXL):

M (please correct me if I am wrong)

Related suggestions: (put links to related suggestions here)

Perhaps we can have a yield version that translates into a scan:

let results =
  [ for s with t in ts do
      yield s + t
  ]

There is some slight ambiguity here, as yield is no longer a required keyword when the for is used in a sequence/list/array. Some users may expect for-with to behave as a scan even without the yield keyword if it is within a list.

Please let me know if I should open another issue for this suggestion, and I will update with the link here.

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• This is not a question (e.g. like one you might ask on StackOverflow) and I have searched StackOverflow for discussions of this issue
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Please tick all that apply:

• This is not a breaking change to the F# language design
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[brianrourkeboll]

This is interesting. I have wondered about something like this before...

Re: adding something like this to the language

For something like this to work in F#, there would need to be some type-based and/or structural definition of "foldable" as there currently is for enumerables.

Even then, types would actually need to conform in their definitions to whatever convention was decided upon — unless this RFC were implemented: https://github.com/fsharp/fslang-design/blob/cd6085fb9f3a50093938d616fde8776d3be2cdad/RFCs/FS-1043-extension-members-for-operators-and-srtp-constraints.md

An alternative approach could be to borrow the idea from C# of using some kind of attribute to tie modules and their fold functions to their corresponding types. C# does this to find the appropriate Create method to use for collection expressions. Again, though, any existing modules would need to be annotated.

Maybe you could use a heuristic like "look for a fold function operating on type T on a module in scope called T or TModule," although that seems like a pretty messy way to go about it.

Computation expressions

It's already possible to implement something like this yourself using computation expressions. All of the problems above still prevent you from writing a more general computation expression that handles any "foldable" type, though (see, e.g., https://github.com/Savelenko/FSharp.Control.Fold?tab=readme-ov-file#making-your-data-types-compatible-with-the-library).

I can think of a few other syntaxes off the top of my head.

Here are some very rough proofs of concept: https://gist.github.com/brianrourkeboll/830408adf29fa35c2d027178b9f08e3c

1. This might be the most consistent:

   fold state { for x in xs -> fun acc -> f acc x }

   let sum xs = fold 0 { for x in xs -> (+) x }
   
   let rev xs = fold [] { for x in xs -> fun acc -> x :: acc }
   
   let rebuild m = fold Map.empty { for KeyValue (k, v) in m -> Map.add k v }

   I.e., you provide the initial state as an argument to the builder, and you then simply yield functions that take the state and return a new value.

2. This is kind of nice, too, though?

   fold state { for acc, x in xs -> f acc x }

   let sum xs = fold 0 { for acc, x in xs -> acc + x }
   
   let rev xs = fold [] { for acc, x in xs -> x :: acc }
   
   let rebuild m = fold Map.empty { for acc, KeyValue (k, v) in m -> acc.Add (k, v) }

   I.e., you provide the initial state as an argument to the builder, and then the intermediate state is exposed to you as the first element of a tuple produced via for.

3. I also kind of like this, although it starts to make less sense if you allow multiple loops, standalone yields, etc.:

   fold { for acc, x in state, xs -> f acc x }

   let sum xs = fold { for acc, x in 0, xs -> acc + x }
   
   let rev xs = fold { for acc, x in [], xs -> x :: acc }
   
   let rebuild m = fold { for acc, KeyValue (k, v) in Map.empty, m -> acc.Add (k, v) }

   I.e., you provide the initial state as an argument to in, and then the intermediate state is exposed to you as the first element of a tuple produced via for.

You can extend any of these to handle additional foldable types, including non-enumerable ones, although you need to do it one by one:

type FoldBuilder<'T> with
    member inline builder.For (option : _ option, [<InlineIfLambda>] body : _ -> FoldBuilderCode<_>) =
        FoldBuilderCode<'T> (fun sm ->
            match option with
            | Some x -> (body x).Invoke &sm
            | None -> ())

let two = fold 1 { for x in Some 1 -> (+) x }

type FoldBuilder<'T> with
    member inline builder.For (result : Result<_, _>, [<InlineIfLambda>] body : _ -> FoldBuilderCode<_>) =
        FoldBuilderCode<'T> (fun sm ->
            match result with
            | Ok x -> (body x).Invoke &sm
            | Error _ -> ())

let three = fold 1 { for x in Ok 2 -> (+) x }

I guess you could also do something like this, although again you'd still need to explicitly define extensions:

[<Extension>]
type FoldBuilderExtensions =
    [<Extension>]
    static member inline For<
        'Input,
        'Extensions,
        'Intermediate,
        'State when ('Input or 'Extensions) : (static member Fold : ('State -> 'Intermediate -> 'State) * 'State * 'Input -> 'State)
        > (
            _builder : FoldBuilder<'State, 'Extensions>,
            foldable : 'Input,
            [<InlineIfLambda>] body : 'Intermediate -> FoldBuilderCode<'State>
        ) =
        let folder sm x =
            let mutable sm = sm
            (body x).Invoke &sm
            sm

        let inline call folder state input = ((^Input or ^Extensions) : (static member Fold : ('State -> 'Intermediate -> 'State) * 'State * 'Input -> 'State) (folder, state, input))

        FoldBuilderCode<'State> (fun sm -> sm <- call folder sm foldable)

[<Extension>]
type FoldExtensions =
    [<Extension>]
    static member Fold (folder, state, input) = List.fold folder state input

    [<Extension>]
    static member Fold (folder, state, input) = Array.fold folder state input

    [<Extension>]
    static member Fold (folder, state, input) = Set.fold folder state input

    [<Extension>]
    static member Fold (folder, state, input) = Option.fold folder state input

    [<Extension>]
    static member Fold (folder, state, input) = ValueOption.fold folder state input

    [<Extension>]
    static member Fold (folder, state, input) = Result.fold folder state input

    [<Extension>]
    static member Fold (folder, state, input) = Seq.fold folder state input

let fold<'T> state = FoldBuilder<'T, FoldExtensions> state

let sum = fold 0 { for x in [1..100] -> (+) x }

let sum' = fold 0 { for x in Some 1 -> (+) x }

let sum'' = fold 0 { for x in set [1..100] -> (+) x }

let sum''' = fold 0 { for x in [|1..100|] -> (+) x }

[reinux]

Thanks for the detailed reply!

The CEs do indeed look nice. I probably wouldn't hesitate to use them myself if they were built-in, but I do have to wonder if it might be a bit magical-looking for beginners?

Maybe instead of unrolling to fold, we can translate it all the way to the imperative version, like this (using IEnumerable as opposed to ICollection too, btw):

let result =
  let mutable s = s
  let enr = ts.GetEnumerator()
  while enr.MoveNext() do
    s <- f s enr.Current
  s

We do lose some rigor, but my gut feeling is that that might not be so bad for a semi-imperative language like F#, as long as it's documented that that's what it's doing under the hood. Unless I'm missing something, it should be fairly rare that fold implementations do anything beyond that, so I think it shouldn't cause too many surprises.

[Happypig375]

isn't the for loop an imperative construct with the iteration variable mutable without the use of the mutable keyword anyway? the benefit of hiding mutability just feels like a lie.

[En3Tho]

I don't really like it from first glance. It seems to introduce a huge change in F# making loops actually return something other than "unit" but it seems to support only "fold" operation which can be executed as a function or a CE like described above. And it's confusing.

let s = 0
let q = for i in 0..10 do s + i // unit
let w = for s with i in 0..10 do s + i // type is int even though loop body is very similar, the only change is at loop declaration

Also, I think it might be quite confusing because today F# is "yielding" expressions in loops
E.g.

let s = 0
let q = seq { for i in 1..10 do s + i } // int seq, 10 values
let w = seq { for s with i in 1..10 do s + i } // int seq, but now only 1 value is yielded

let q1 = seq { for i in 1..10 do s + i; s + i } // int seq, 20 values
let w2 = seq { for s with i in 1..10 do s + i; s + i } // int seq, 11 values now?

[Happypig375]

@En3Tho

// Top code
let s = 0
let q = for i in 0..10 do s + i // <- warning (discard), so it already reminds you what it does
let w = for s with i in 0..10 do s + i // value is indeed used

For the bottom code there are a few choices that can be made -

• Allow implicit yields before the final value
• Warn with implicit yield behaviour
• Warn with discard behaviour
• Error because it's ambiguous

Warning with discard is also a valid choice when inside this kind of for loop. Explicit yields are always available if needed.

[Lanayx]

I agree with @En3Tho that suggested syntax is a bit confusing, however I do agree that variable leak is kind of a problem. But I feel it's more a block scope that is missing rather than accumulator, so this syntax reads more natural to me:

for i in 1..10 with mutable s = 0 do
    s <- s + i

I do expect iterator variable to go after for rather than accumulator.

[Happypig375]

@En3Tho @Lanayx Please review the revised version of this suggestion with a full critique on existing approaches of mutable variables or folds with lambdas: fsharp/fslang-design#807

[T-Gro]

Is there a language where programmers can do folds over collections into a single value, without using a fold-like function call?
The closest similarity I could find is using .. = ref initialState in for ... in ocaml:

let acc = ref 0 in for i = 0 to 2 do acc := !acc +i done; !acc

Which in F# one could also rework into mutable instead of ref:

let mutable sum = 0 in 
    for x in [|1;2;3|] do 
        sum <- sum + x

(which is not a style typically used in F# programs)

[Happypig375]

@T-Gro

// Current - fold
let effects, model =
    Seq.fold (fun (effects, model) item ->
        let effect, model = Model.action item model // Pure function
        let model = Model.action2 model // Pure function
        effect :: effects, model)
        ([], model) // ugh - hard to get order and parentheses right
        items
// Current - ||> fold
let effects, model =
    (([], model), items) // ugh - nested tuples and hard to get order and parentheses right
    ||> Seq.fold (fun (effects, model) item ->
        let effect, model = Model.action item model // Pure function
        let model = Model.action2 model // Pure function
        effect :: effects, model)
// Current - recursive function
let effects, model =
    let rec [<TailCall>] processItems (effects, model) items = // boilerplate, hard to write correctly
        match items with // boilerplate and specific to lists - can't do this for other collections
        | [] -> effects, model // boilerplate
        | item::items -> // boilerplate
            let effect, model = Model.action item model // Pure function
            let model = Model.action2 model // Pure function
            processItems (effect :: effects, model) items
    processItems ([], model) items
// Current - mutable accumulator
let effects, model =
    let mutable accum = [], model // hmm, a mutable variable is required even in an architecture of pure functions and it needs the most boilerplate
    for item in items do
        let effects, model = accum // boilerplate
        let effect, model = Model.action item model // Pure function
        let model = Model.action2 model // Pure function
        accum <- effect :: effects, model
    accum // boilerplate
// Proposed - fold loop (simplest this can be)
let effects, model =
    for effects, model = [], model with item in items do
        let effect, model = Model.action item model // Pure function
        let model = Model.action2 model // Pure function
        effect :: effects, model

[T-Gro]

I get the benefits and how it fits pragmatic programming, my question was really after if this was being solved in any other programming language or its library ecosytem.

Also not really sold on the ordering of the syntax - I try to think of it as evolution of for item in items, and have question to the choices behind the proposed syntax:

Why does the accumulated state syntactically come before the enumeration?
Why does the binding come before the with keyword, and not the other way around?

Is the natural idea of a fold primarily about the state, "with" the enumeration being the secondary? I tend to think about it being iteration first, "with" some additional state to be kept around.

(in natural language, I would typically say "The code is folding over ...collection... while using ...state accumulation...")

[Happypig375]

any other programming language or its library ecosytem.

It seems that other programming languages and their library ecosystems maintain the bifurcation of either loops with mutable accumulators or folds with lambda body.

order of accumulator before enumeration

The accumulator is placed before the enumeration item because enumeration state must exist before it is used to get an item from the sequence. It is also why the fold lambda body takes the accumulator before the sequence item, making refactors from fold easy.

let sequence = ["Hello"; " "; "World"; "!"] // sequence
let mutable state_accum = "" // accumulator state
let state_iterator = sequence.GetEnumerator() // enumerator state
while state_iterator.MoveNext() do
    let word = state_iterator.Current // state must be defined before the iteration item is retrieved
    state_accum <- state_accum + word
printfn "%s" state_accum

This also highlights that while for i = 1 to 10 do and for i in 1 .. 10 do enumerate on the same items, they are semantically very different: for i = 1 to 10 do has i as the enumeration state, while for i in 1 .. 10 do hides the enumeration state and i is the enumeration item. Meanwhile, for i = 1 with j in 1 .. 10 do has two enumeration states: i and the enumerator of 1 .. 10 which must exist before j is retrieved. Therefore, it makes less sense to put the accumulator after the enumeration item.

I accepted placing the accumulator after the enumerator in the RFC: fsharp/fslang-design#807

[brianrourkeboll]

I am personally still not sold on this feature given how much overlap it has with what you can already do with computation expressions.

The arguments in the RFC against the suitability of computation expressions for this hold equally well for literally any use of computation expressions for any purpose.

To me, that means that we might be better served by investing time in addressing those pain points of computation expressions generally—e.g., an inline bind operator (#1070), improvements to the compiler that take advantage of knowledge of how computation expression builder methods are used to give better error messages or to resolve overload resolution ambiguities, etc.—rather than adding new special syntax for specific use-cases like this.

The RFC also currently seems to focus on an elaboration of the new syntax to something involving for-loops. But as noted above in #1362 (comment), there are many foldable things that are not directly enumerable with a regular for-loop in F# today—like 'T option, Result<'Ok, 'Error>, etc.—and F# of course currently lacks a way to define a generic "foldable" construct at all, whether for consumption by computation expressions or by a dedicated syntax.

[Happypig375]

@brianrourkeboll

The arguments in the RFC against the suitability of computation expressions for this hold equally well for literally any use of computation expressions for any purpose.

• "an inline bind operator (Inline bind operator for computation expressions #1070)" is unrelated to the point about "But this is not orthogonal to an existing computation expression context unlike the for loop which allows let! inside that refer to the outer context."
  Consider this:

  // CE (variation 1)
  let xs = [ fold 0 { for x in xs do yield 1 (*You cannot yield to the outer list here!*); yield (+) x } ]
  // Proposed fold loop (Alternative 2)
  let xs = [ for s = 0 with x in xs do yield 1 (* This yields to outer list :D *); s + x; done; s ]

• The point about "Computation expressions also show a heavily different syntax compared to for loops and folds which add hinderance to understanding" is also not applicable to "any use of computation expressions for any purpose" - it is fine to use the async or task CEs for example to hide large chunks of logic. But for something as simple as an accumulator, using a CE is overkill with the heavy semantics brought in with braces and indirection to underlying code (one must follow each Bind call to understand what a let! really does, combine with all the other methods in a CE) compared to just writing the underlying code (also applicable to option and result CEs for example).

there are many foldable things that are not directly enumerable with a regular for

Indeed. The standard method to represent a foldable type is to implement seq. Making options implement seq was declined because of the use of null to represent None, so Option.toSeq must be called. This shouldn't be a problem for ValueOption or Result though.

[Happypig375]

@brianrourkeboll See if the updated explanation here fsharp/fslang-design@979c708 is better.