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bishaboshabishabosha
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better explain message for 'pattern expected' (#18753)
2 parents 64134b3 + 721dcad commit 15033c7

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‎compiler/src/dotty/tools/dotc/reporting/messages.scala‎

Lines changed: 22 additions & 25 deletions
Original file line numberDiff line numberDiff line change
@@ -973,66 +973,63 @@ extends SyntaxMsg(IllegalStartOfSimplePatternID) {
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def msg(using Context) = "pattern expected"
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def explain(using Context) = {
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val sipCode =
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"""def f(x: Int, y: Int) = x match {
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| case `y` => ...
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|}
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"""
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"""def f(x: Int, y: Int) = x match
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| case `y` => ...""".stripMargin
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val constructorPatternsCode =
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"""case class Person(name: String, age: Int)
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|
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|def test(p: Person) = p match {
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| case Person(name, age) => ...
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|}
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"""
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val tupplePatternsCode =
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"""def swap(tuple: (String, Int)): (Int, String) = tuple match {
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| case (text, number) => (number, text)
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|}
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"""
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| def test(p: Person) = p match
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| case Person(name, age) => ...""".stripMargin
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val tuplePatternsCode =
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"""def swap(tuple: (String, Int)): (Int, String) = tuple match
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| case (text, number) => (number, text)""".stripMargin
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val patternSequencesCode =
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"""def getSecondValue(list: List[Int]): Int = list match {
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| case List(_, second, x:_*) => second
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| case _ => 0
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|}"""
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"""def getSecondValue(list: List[Int]): Int = list match
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| case List(_, second, x*) => second
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| case _ => 0""".stripMargin
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i"""|Simple patterns can be divided into several groups:
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|- Variable Patterns: ${hl("case x => ...")}.
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|- Variable Patterns: ${hl("case x => ...")} or ${hl("case _ => ...")}
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| It matches any value, and binds the variable name to that value.
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| A special case is the wild-card pattern _ which is treated as if it was a fresh
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| variable on each occurrence.
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|
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|- Typed Patterns: ${hl("case x: Int => ...")} or ${hl("case _: Int => ...")}.
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|- Typed Patterns: ${hl("case x: Int => ...")} or ${hl("case _: Int => ...")}
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| This pattern matches any value matched by the specified type; it binds the variable
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| name to that value.
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|
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|- Literal Patterns: ${hl("case 123 => ...")} or ${hl("case 'A' => ...")}.
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|- Given Patterns: ${hl("case given ExecutionContext => ...")}
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| This pattern matches any value matched by the specified type; it binds a ${hl("given")}
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| instance with the same type to that value.
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|
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|- Literal Patterns: ${hl("case 123 => ...")} or ${hl("case 'A' => ...")}
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| This type of pattern matches any value that is equal to the specified literal.
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|
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|- Stable Identifier Patterns:
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|
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| $sipCode
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| ${hl(sipCode)}
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|
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| the match succeeds only if the x argument and the y argument of f are equal.
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|
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|- Constructor Patterns:
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|
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| $constructorPatternsCode
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| ${hl(constructorPatternsCode)}
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|
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| The pattern binds all object's fields to the variable names (name and age, in this
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| case).
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|
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|- Tuple Patterns:
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|
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| $tupplePatternsCode
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| ${hl(tuplePatternsCode)}
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|
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| Calling:
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|
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| ${hl("""swap(("Luftballons", 99)""")}
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| ${hl("""swap(("Luftballons", 99))""")}
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|
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| would give ${hl("""(99, "Luftballons")""")} as a result.
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|
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|- Pattern Sequences:
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|
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| $patternSequencesCode
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| ${hl(patternSequencesCode)}
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|
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| Calling:
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|

‎tests/neg/i18750-format.check‎

Lines changed: 68 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,68 @@
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-- [E032] Syntax Error: tests/neg/i18750-format.scala:4:7 --------------------------------------------------------------
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4 | case # => () // error
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| ^
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| pattern expected
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|---------------------------------------------------------------------------------------------------------------------
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| Explanation (enabled by `-explain`)
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|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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| Simple patterns can be divided into several groups:
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| - Variable Patterns: case x => ... or case _ => ...
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| It matches any value, and binds the variable name to that value.
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| A special case is the wild-card pattern _ which is treated as if it was a fresh
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| variable on each occurrence.
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|
14+
| - Typed Patterns: case x: Int => ... or case _: Int => ...
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| This pattern matches any value matched by the specified type; it binds the variable
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| name to that value.
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|
18+
| - Given Patterns: case given ExecutionContext => ...
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| This pattern matches any value matched by the specified type; it binds a given
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| instance with the same type to that value.
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|
22+
| - Literal Patterns: case 123 => ... or case 'A' => ...
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| This type of pattern matches any value that is equal to the specified literal.
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|
25+
| - Stable Identifier Patterns:
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|
27+
| def f(x: Int, y: Int) = x match
28+
| case `y` => ...
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|
30+
| the match succeeds only if the x argument and the y argument of f are equal.
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|
32+
| - Constructor Patterns:
33+
|
34+
| case class Person(name: String, age: Int)
35+
|
36+
| def test(p: Person) = p match
37+
| case Person(name, age) => ...
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|
39+
| The pattern binds all object's fields to the variable names (name and age, in this
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| case).
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|
42+
| - Tuple Patterns:
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|
44+
| def swap(tuple: (String, Int)): (Int, String) = tuple match
45+
| case (text, number) => (number, text)
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|
47+
| Calling:
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|
49+
| swap(("Luftballons", 99))
50+
|
51+
| would give (99, "Luftballons") as a result.
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|
53+
| - Pattern Sequences:
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|
55+
| def getSecondValue(list: List[Int]): Int = list match
56+
| case List(_, second, x*) => second
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| case _ => 0
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|
59+
| Calling:
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|
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| getSecondValue(List(1, 10, 2))
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|
63+
| would give 10 as a result.
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| This pattern is possible because a companion object for the List class has a method
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| with the following signature:
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|
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| def unapplySeq[A](x: List[A]): Some[List[A]]
68+
---------------------------------------------------------------------------------------------------------------------

‎tests/neg/i18750-format.scala‎

Lines changed: 4 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,4 @@
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//> using options -explain
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def test = 23 match
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case # => () // error

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