Allow assigning arbitrary values when pattern matching

[Larocceau]

I propose we make it possible to assign arbitrary values in match statements, so that OR matching becomes a bit more versatile by allowing you to define default values for valueless cases; eg:

type myType =
    | CaseWithValue of int
    | CaseWithoutValue
    
let myFuction =
    function
    | CaseWithValue v
    | CaseWithoutValue with let v = 1 ->
        v + 1

The existing way of approaching this problem in F# is to handle the cases separately

type myType =
    | CaseWithValue of int
    | CaseWithoutValue
    
let myFuction =
    function
    | CaseWithValue v ->  v + 1
    | CaseWithoutValue -> 2 

Pros and Cons

The advantages of making this adjustment to F# are the*

• We will be able to make better use of OR matching.

The disadvantages of making this adjustment to F# are

• This will use up a keyword

Extra information

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

Related suggestions: (put links to related suggestions here)

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Please tick these items by placing a cross in the box:

• [x] This is not a question (e.g. like one you might ask on StackOverflow) and I have searched StackOverflow for discussions of this issue
• [x] This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to the compiler and tooling repository
• [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it
• [x] I have searched both open and closed suggestions on this site and believe this is not a duplicate

Please tick all that apply:

• [x] This is not a breaking change to the F# language design
• [x] I or my company would be willing to help implement and/or test this

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[brianrourkeboll]

One existing way would probably be to use an active pattern and an & or as pattern:

let f x =
    let (|Otherwise|) _ = 1

    match x with
    | A v
    | B & Otherwise v -> v + 1

[Tarmil]

@brianrourkeboll You can even pass the value as argument to this active pattern.

let (|With|) v _ = v

let f x =
    match x with
    | A v
    | B & With 1 v -> v + 1

[edgarfgp]

I think parameterless total active patterns should only be allowed on functions ? See https://github.com/dotnet/fsharp/issues/17190

[LyndonGingerich]

The disadvantages of making this adjustment to F# are

• This will use up a keyword

Do you mean let or with? Both are already reserved keywords.

[voronoipotato]

Merging cases that aren't the same is a dangerous thing to do, just make a function and reuse it. I don't think we should do this and I think adding it would be damaging to the language.

module MyType = 
  let getValueOrDefault defaultValue m =
    match m with
    | CaseWithValue a -> a
    | CaseWithoutValue -> defaultValue 

[T-Gro]

I am marking this as probably not - the examples given here show that existing F# syntax can express the desired intent.

https://github.com/fsharp/fslang-suggestions/issues/1378#issuecomment-2299136473 https://github.com/fsharp/fslang-suggestions/issues/1378#issuecomment-2291487458

Allowing arbitrary expressions after with let v = ... would also further complicate pattern match compilation and related optimizations. The size estimate would have to be bigger here.