roc: Destructuring tag with `=` generates parsing error

This currently produces an ARGUMENTS BEFORE EQUALS parsing error, but it should be parsed as a valid def (when used either as a top-level declaration or in an expression):

Email str = Email "blah@example.com"

It should put str in scope and set it equal to "blah@example.com" - just like if I did this:

{ str } = { str: "blah@example.com" }

About this issue

Original URL
State: closed
Created 3 years ago
Comments: 15 (15 by maintainers)

Most upvoted comments

those are the three places a pattern can syntactically occur, yes

folkertdev on Dec 31, 2021

Nope! In this case generalization is not a concern, since the “email” parameter in the function definition is not under a let binding before it accessed in the body. Changing it to be a type error is done the same way as we did in #2299

See #2305, which contains a fix for the parsing error and changes the above to a type error 😃

ayazhafiz on Dec 31, 2021

The last line will now cause a typechecking error, but removing Email str = email will fix the type error. There are also ways to fix this (i.e. do a structural subtyping check, like e.g. TypeScript would do).

I see this as a separate question from how = should affect type inference - for example, if I have a function that accepts a [ True, False, Blah ], should I be able to give a Bool to it? I can see arguments for and against that design question, but I think it’s a separate issue! 😄

rtfeldman on Dec 30, 2021