Allow typevars in do expressions
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Allows constructs like f() do (x::T) where T; body end It's already being parsed, but the where clause is put into a tuple, causing a syntax error downstream. This change just avoids putting it in a tuple.
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I think it is best for proposed syntax changes to start as or have an associated issue in the JuilaLang/julia repo for viability. |
LilithHafner
previously approved these changes
Jul 18, 2024
| "f(x) do y body end" => "(call f x (do (tuple y) (block body)))" | ||
| "@f(x) do y body end" => "(macrocall-p @f x (do (tuple y) (block body)))" | ||
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| "f(x) do (y::T) where T body end" => "(call f x (do (where (parens (::-i y T)) T) (block body)))" |
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Suggested change
| "f(x) do (y::T) where T body end" => "(call f x (do (where (parens (::-i y T)) T) (block body)))" | |
| "f(x) do y::T where T body end" => "(call f x (do (tuple (::-i y (where T T))) (block body)))" | |
| "f(x) do (y::T) where T body end" => "(call f x (do (where (parens (::-i y T)) T) (block body)))" |
Maybe add a test for the case without parenthesis (IIUC the where clause will bind tighter than the ::).
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Actually, as @c42f points out, the relative precedence of :: and where is already context dependent:
julia> parsestmt(SyntaxNode, "function f()::T where T 0 end")
line:col│ tree │ file_name
1:1 │[function]
1:10 │ [where]
1:10 │ [::-i]
1:10 │ [call]
1:10 │ f
1:15 │ T
1:23 │ T
1:24 │ [block]
1:25 │ 0
julia> parsestmt(SyntaxNode, "f()::T where T = 0")
line:col│ tree │ file_name
1:1 │[=]
1:1 │ [::-i]
1:1 │ [call]
1:1 │ f
1:6 │ [where]
1:6 │ T
1:14 │ T
1:18 │ 0So in this case, I think we should make :: bind tighter than where so parentheses are not necessary here.
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See also: JuliaLang/julia#55159 And also, as a syntax change this would need to be gated on VERSION. |
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(@c42f edit:) Upstream syntax discussion: JuliaLang/julia#54915
There are several ways to create functions in julia. One of them is the
dosyntax. You can't dispatch ondofunctions, but now and then the need comes up to capture the types of variables. Although this is easy withtypeofinside the function body, it turns out to be quite easy to enable the use ofwhereclauses in thedosyntax. This would make adofunction similar to the other function definitions.The reason it's easy is that such expressions are already being parsed, but a minor detail prevents it from being accepted as a valid expression:
The
whereclause becomes encapsulated in atuple. The reason is that after the parser seesdo, it looks for a comma separated list. After the list is parsed, i.e. on;or linefeed, the list is encapsulated in atuple. This means that thewhereclause becomes an argument to the function, which fails as a syntax error. By modifying the parsing ofdoexpressions to avoid insertingtupleif the newly parsed list is awhere, the parse result becomes:This is a valid
:->expression, just like an anonymous function definition((x::T) where T) -> T(x)currently is.The PR is a one-line change to
parser.jl, isolated to parsing what followsdo. It does not enable any new or very useful functionality, it merely will make functions defined indoquite similar to other function definitions.I don't think this change will break anything, as the construction currently results in a
ERROR: syntax: ...failure.