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spec: document that x[i,] (trailing comma) is permitted #55007

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adonovan opened this issue Sep 11, 2022 · 3 comments
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spec: document that x[i,] (trailing comma) is permitted #55007

adonovan opened this issue Sep 11, 2022 · 3 comments
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Documentation FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
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Go1.20

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@adonovan
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adonovan commented Sep 11, 2022 •
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The spec does not seem to permit a comma after the index in an expression such as a[i,], yet both the compiler and the go/parser package permit it, presumably because without type information it's impossible to distinguish an array index expression a[i,] from a parameterized type such as M[K,V,] where a comma is permitted after the list of instantiating types. The spec should probably be updated to say that a comma is allowed but ignored. The alternative, of having the type checker allow the comma in a type but reject it in a term, would require recording the presence of the comma in the syntax tree, which seems undesirable.

https://go.dev/play/p/JcES0elTHiG
@griesemer griesemer added the NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. label Sep 11, 2022
@griesemer griesemer added this to the Go1.20 milestone Sep 11, 2022
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griesemer commented Sep 11, 2022 •
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Your analysis is correct. Array/slice index expressions do not permit a trailing comma, but instantiations do because they follow the syntax of (type) parameter lists, which generally allow a trailing comma.

The other approach is to consider an expression of the form x[t,] (with a trailing comma) always as instantiation, i.e., "foo"[0,] would be a type error ("foo" is not a generic type, 0 is not a type, etc.).

Related: https://twitter.com/robertgriesemer/status/1283142956153573376

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@ianlancetaylor agrees that we should not report a syntax error during type checking. I will update the spec accordingly. Notably, gofmt will remove any such trailing commas.

@griesemer griesemer added Documentation and removed NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. labels Nov 15, 2022
@griesemer griesemer changed the title spec: is x[i,] legal? spec: document that x[i,] (trailing comma) is permitted Nov 15, 2022
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Change https://go.dev/cl/452618 mentions this issue: spec: document that trailing comma is valid after index in index expressions

@dmitshur dmitshur added the NeedsFix The path to resolution is known, but the work has not been done. label Nov 23, 2022
@golang golang locked and limited conversation to collaborators Nov 23, 2023
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