F# has used expr.[idx] as indexing syntax. This was inherited from OCaml, which uses it for string indexing lookup. However in F# the notation has always been type-directed, so a well-known strong indexable type has been needed for expr.

Based on the experiment at dotnet/fsharp#11749 I propose we allow expr[idx] as indexer/slicing syntax and normalize this as the preferred indexing syntax from F# 6.0 onwards.

This is to make learning F# easier and allow simpler migration of existing code.

As with any indexing today known type information is required, e.g.

let f (arg: int[]) = arg[0], arg[1]

This is because

let f arg = arg[0]

exists today in F# and infers x to be a function x: int list -> 'a. In this case we woll give a deprecation warning on x[0] to say "please add a space indicating function application or a type annotation indicating indexing" i.e. let f arg = arg [0] for the former.

Technically this is not a breaking change to existing well-typed code (because we can use types to disambiguate between a function and something supporting indexing). However, as a matter of policy we will emit warnings to recommend small adjustments to existing code to properly disambiguate, by introducing spaces for function application. Informational recommendations to change expr.[idx] to expr[idx] will also be given.

I will put the details in an RFC

Extra information

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

Related suggestions: (put links to related suggestions here)

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• I have searched both open and closed suggestions on this site and believe this is not a duplicate
• This is not something which has obviously "already been decided" in previous versions of F#. actually it is and we're looking at it to ease language learning

Please tick all that apply:

• This is not a breaking change to the F# language design. it's not a breaking change but we will use warnings to recommend small adjustments to existing code to properly disambiguate
• I or my company would be willing to help implement and/or test this

For Readers

If you would like to see this issue implemented, please click the 👍 emoji on this issue. These counts are used to generally order the suggestions by engagement.