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Treat - as negative operator immediately after opening bracket instead of infix minus #277

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Merged
jfmengels merged 4 commits into from
Apr 3, 2026
Merged

Treat - as negative operator immediately after opening bracket instead of infix minus #277

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aad6fdd
Treat `-` as negative operator immediately after opening bracket instead
dillonkearns Apr 2, 2026
8d5229d
Treat `-` as negative operator immediately after `=` in record fields.
dillonkearns Apr 2, 2026
200b74b
Parse negative numbers immediately after then and else keywords.
dillonkearns Apr 2, 2026
02d0c38
Parse negative numbers immediately after `->` in case expressions.
dillonkearns Apr 2, 2026
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59 changes: 58 additions & 1 deletion src/Elm/Parser/Expression.elm
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Original file line number Diff line number Diff line change
Expand Up @@ -944,12 +944,35 @@ negationOperation =
"," ->
negationAfterMinus

"[" ->
negationAfterMinus

"=" ->
negationAfterMinus

-- from the end of -> in case branches
">" ->
Comment thread
jfmengels marked this conversation as resolved.
negationAfterMinus

-- TODO only for tests
"" ->
negationAfterMinus

_ ->
negationWhitespaceProblem
-- Check if preceded by a keyword that introduces an expression.
-- We check for the keyword AND a space/boundary before it
-- to avoid matching identifiers that end with the keyword
-- (e.g., "within" should not match "in").
if
endsWithKeyword "then" offset source
|| endsWithKeyword "else" offset source
|| endsWithKeyword "of" offset source
|| endsWithKeyword "in" offset source
then
negationAfterMinus

else
negationWhitespaceProblem
)
)

Expand All @@ -959,6 +982,40 @@ negationWhitespaceProblem =
ParserFast.problem "if a negation sign is not preceded by whitespace, it's considered subtraction"


{-| Check if the source immediately before the minus sign at `offset - 1`
ends with the given keyword, and that keyword is preceded by a word boundary
(space, newline, or start of input) so we don't match identifiers like
"within" for keyword "in".
-}
endsWithKeyword : String -> Int -> String -> Bool
endsWithKeyword keyword offset source =
let
keywordLen =
String.length keyword

keywordStart =
offset - 1 - keywordLen
in
(String.slice keywordStart (offset - 1) source == keyword)
&& (keywordStart <= 0 || isWordBoundary (String.slice (keywordStart - 1) keywordStart source))


isWordBoundary : String -> Bool
isWordBoundary char =
case char of
" " ->
True

"\n" ->
True

"\u{000D}" ->
True

_ ->
False


negationAfterMinus : Parser (WithComments (Node Expression))
negationAfterMinus =
ParserFast.map
Expand Down
72 changes: 72 additions & 0 deletions tests/Elm/Parser/ExpressionTests.elm
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Expand Up @@ -566,6 +566,78 @@ all =
(Node { start = { row = 1, column = 3 }, end = { row = 1, column = 4 } } (Integer 1))
)
)
, test "negated expression in list literal without spaces" <|
\() ->
"[-3, 5, -1]"
|> expectAst
(Node { start = { row = 1, column = 1 }, end = { row = 1, column = 12 } }
(ListExpr
[ Node { start = { row = 1, column = 2 }, end = { row = 1, column = 4 } }
(Negation (Node { start = { row = 1, column = 3 }, end = { row = 1, column = 4 } } (Integer 3)))
, Node { start = { row = 1, column = 6 }, end = { row = 1, column = 7 } }
(Integer 5)
, Node { start = { row = 1, column = 9 }, end = { row = 1, column = 11 } }
(Negation (Node { start = { row = 1, column = 10 }, end = { row = 1, column = 11 } } (Integer 1)))
]
)
)
, test "negated expression in record literal without spaces" <|
\() ->
"{a=-1}"
|> expectAst
(Node { start = { row = 1, column = 1 }, end = { row = 1, column = 7 } }
(RecordExpr
[ Node { start = { row = 1, column = 2 }, end = { row = 1, column = 6 } }
( Node { start = { row = 1, column = 2 }, end = { row = 1, column = 3 } } "a"
, Node { start = { row = 1, column = 4 }, end = { row = 1, column = 6 } }
(Negation (Node { start = { row = 1, column = 5 }, end = { row = 1, column = 6 } } (Integer 1)))
)
]
)
)
, test "negated expression after arrow without space" <|
\() ->
"case x of\n _->-1"
|> expectAst
(Node { start = { row = 1, column = 1 }, end = { row = 2, column = 10 } }
(CaseExpression
{ expression =
Node { start = { row = 1, column = 6 }, end = { row = 1, column = 7 } } (FunctionOrValue [] "x")
, cases =
[ ( Node { start = { row = 2, column = 5 }, end = { row = 2, column = 6 } } AllPattern
, Node { start = { row = 2, column = 8 }, end = { row = 2, column = 10 } }
(Negation (Node { start = { row = 2, column = 9 }, end = { row = 2, column = 10 } } (Integer 1)))
)
]
}
)
)
, test "negated expression after then without space" <|
\() ->
"if True then-1 else 0"
|> expectAst
(Node { start = { row = 1, column = 1 }, end = { row = 1, column = 22 } }
(IfBlock
(Node { start = { row = 1, column = 4 }, end = { row = 1, column = 8 } } (FunctionOrValue [] "True"))
(Node { start = { row = 1, column = 13 }, end = { row = 1, column = 15 } }
(Negation (Node { start = { row = 1, column = 14 }, end = { row = 1, column = 15 } } (Integer 1)))
)
(Node { start = { row = 1, column = 21 }, end = { row = 1, column = 22 } } (Integer 0))
)
)
, test "negated expression after else without space" <|
\() ->
"if True then 0 else-1"
|> expectAst
(Node { start = { row = 1, column = 1 }, end = { row = 1, column = 22 } }
(IfBlock
(Node { start = { row = 1, column = 4 }, end = { row = 1, column = 8 } } (FunctionOrValue [] "True"))
(Node { start = { row = 1, column = 14 }, end = { row = 1, column = 15 } } (Integer 0))
(Node { start = { row = 1, column = 20 }, end = { row = 1, column = 22 } }
(Negation (Node { start = { row = 1, column = 21 }, end = { row = 1, column = 22 } } (Integer 1)))
)
)
)
, test "negated expression for value" <|
\() ->
"-x"
Expand Down