non symmetricity between `fun` and `function` for tuple forces usage of parens

[smoothdeveloper]

those are allegedly similar semantically:

fun a -> //...
function a -> //...

but for tuple, it doesn't work

fun a,b -> // error FS0010: Unexpected symbol ',' in lambda expression. Expected '->' or other token.
function a,b -> //...

I wonder if this can be fixed without ambiguity, because it would be an incomplete lambda in current version of the language and it would reestablish fuller symmetry with function construct.

The existing way of approaching this problem in F# is to use function or wrap with parens.

Maybe a code fix for putting the parens, or turning into function would make sense if we can't make this suggestion work / it is rejected on some grounds.

Pros and Cons

The advantages of making this adjustment to F# are semantically more consistent.

The disadvantages of making this adjustment to F# are may prevent other constructs in the future.

Extra information

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

Related suggestions: (put links to related suggestions here)

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• [x] This is a language change and not purely a tooling change.
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Please tick all that apply:

• [x] This is not a breaking change to the F# language design
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[Martin521]

fun and function are actually different. fun expects a parameter list and a function body (where each parameter must be enclosed in parentheses if it is a pattern or has a type annotation), in the same way as a named function. function expects match expressions, separated by '|'. So the mentioned behavior makes sense.

[Martin521]

Actually, I think the use of function should be discouraged in favour of proper matches, except for one-liners. But for function one-liners you get a no-no from Fantomas :-(

[realparadyne]

No-no from me on discouraging the use of function. They are brilliant for multiple step transformations starting with a regular match to transform something and then doing another match on that result without having to think up a name to bind the intermediate result(s) to.

Basically, anywhere that an expression result needs to be the 'function' of some singular input value that can have multiple shapes. Without having to come up with a name and use it twice, redundantly, both times.

[realparadyne]

No-no from me on discouraging the use of function. They are brilliant for multiple step transformations starting with a regular match to transform something and then doing another match on that output without having to think up a name to bind the intermediate result(s) to.

Basically anywhere that an expression result needs to be the 'function' of some input value that can have multiple shapes.

[konst-sh]

to me also the tuple between fun and -> looks unambiguous enough to interpret it as function argument pattern, but it may cause problems if fun will become optional #168

[LyndonGingerich]

those are allegedly similar semantically:

fun a -> //...
function a -> //...

Could you explain? To the best of my knowledge, the second line is invalid.

[smoothdeveloper]

@LyndonGingerich in such case:

let data = [1,"1";2,"2"]
data |> List.map (function a,b -> a * 2)
data |> List.map (fun (a,b) -> a * 2)