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Record-like member constraints #717

cannorin opened this Issue Jan 24, 2019 · 0 comments


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cannorin commented Jan 24, 2019

Record-like member constraints

I propose we allow record-like syntax sugar for member constraints:

let inline f (x: ^TX when ^TX :> {| X: ^X; Y: ^Y1 -> ^Y2 |}) = ...

instead of

let inline f (x: ^TX when ^TX: (member X: ^X) and ^TX: (member Y: ^Y1 -> ^Y2)) = ...

Pros and Cons


  • It makes member constraints easier to read and write, especially when we have many member constraints to one type.

  • Combined with (anonymous) records, it can emulate structural subtyping with clean syntax:

let inline getX (x: ^TX when ^TX :> {| X: ^X |}) = ...

getX {| X = 0 |}
getX {| X = 1; Y = "foo" |}


  • It's very confusing with some existing features.
    • It uses the same syntax with anonymous records and type constraints.
      • maybe we should add some prefix like ^TX :> interface {| X: ^X |} or ^TX :> shape {| X: ^X |}
      • or borrow the syntax proposed in #483: ^TX : #{| X: ^X |}
    • Although it resembles (anonymous) records, it can actually be used to arbitrary types i.e. classes.
  • It is nearly useless unless we have #440. We still have to write (^TX: (member X: ...) x) to access the fields and methods, which itself is enough to assert the required member constraint.

Extra information

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

Related suggestions: (put links to related suggestions here) #440 #641 #483

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#. 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.

Please tick all that apply:

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