Improve error reporting: Accidental use of , instead of ; as collection separator

[isaacabraham]

What

The following error message is generated from this code sample: -

let x = [ 1, 2, 3, 4, 5 ]
x |> List.map(fun x -> x + 5)

error FS0001: The type 'int' does not match the type 'int * int * int * int * int'

Why

Users new to F# often accidentally use , instead of ; for item separators as this is what they are used to. This is valid F# as you simply get a list with a single tupled item. You will probably get an compiler error (or an exception if you have simply indexed into the list) further down the line as you try to get at the "elements" of it. The user then has to work backwards to realise that they should have used ; instead of ,.

How

This is open to debate as to how this can consistently (and easily) be warned against. It's probably too late to do this where the real error is occurring, because (I would imagine) that by then all you have is a tuple with no context as to where it came from.

Perhaps an alternative approach would be to create a new warning which would be fired if the compiler identifies list with a single Tuple - happy to get ideas on this.

[smoothdeveloper]

A warning asking to put a single literal tuple in a list between parens could solve the problem, but I'll probably find it annoying at times (despite it makes code clearer whenever it used).

let a = [1,2,3,4]

warning: If you intend to use a tuple, please make it explicit by wrapping with parens: let a = [(1,2,3,4)], if you intend this to be several elements, use ';' instead of ','.

[isaacabraham]

That could work. I'm almost thinking that there could (should?) be a "newb mode" compiler flag which would activate warnings like this which are extremely useful for the beginning but could be turned off for the more experienced dev.

[forki]

Please please please no flag. We need to make the error messages good for everyone.

[isaacabraham]

:-) But this is the sort of warning I can see not appealing to everyone. There may be valid use cases for a list with a single item in it which happens to be a Tuple. I'm struggling to think of one - you could (should?) probably use an Option instead.

[forki]

yes that's why we should not do that here ;-)

singleton tuples in lists are perfectly valid use case.

[forki]

But maybe we can actually do something on the side of the error. I'm not completely sure what we actually have in context at this point, but maybe we find something interesting when we look deeper.

[isaacabraham]

But you agree that there is a valid issue in terms of people using , instead of ;?

[forki]

yes. It even happens to people that do F# for very long time ;-)

[isaacabraham]

Not just me then

[dsyme]

Yes, this is a valid usability concern, the "Why" explanation above is very clear.

[smoothdeveloper]

I think single item list with tuples benefits the warning asking to enclose those with (), really makes intent clearer IMHO.