feat: match_expr parsers

src/Lean/Parser/Do.lean

@@ -50,7 +50,7 @@ def notFollowedByRedefinedTermToken :=
   -- but we include them in the following list to fix the ambiguity where
   -- an "open" command follows the `do`-block.
   -- If we don't add `do`, then users would have to indent `do` blocks or use `{ ... }`.
-  notFollowedBy ("set_option" <|> "open" <|> "if" <|> "match" <|> "let" <|> "have" <|>
+  notFollowedBy ("set_option" <|> "open" <|> "if" <|> "match" <|> "match_expr" <|> "let" <|> "let_expr" <|> "have" <|>
       "do" <|> "dbg_trace" <|> "assert!" <|> "for" <|> "unless" <|> "return" <|> symbol "try")
     "token at 'do' element"
 
@@ -150,6 +150,10 @@ def doMatchAlts := ppDedent <| matchAlts (rhsParser := doSeq)
   "match " >> optional Term.generalizingParam >> optional Term.motive >>
   sepBy1 matchDiscr ", " >> " with" >> doMatchAlts
 
+def doMatchExprAlts := ppDedent <| matchExprAlts (rhsParser := doSeq)
+@[builtin_doElem_parser] def doMatchExpr := leading_parser:leadPrec
+  "match_expr " >> optional ("(" >> nonReservedSymbol "meta" >>  " := " >> nonReservedSymbol "false" >> ") ") >> termParser >> " with" >> doMatchExprAlts
+
 def doCatch      := leading_parser
   ppDedent ppLine >> atomic ("catch " >> binderIdent) >> optional (" : " >> termParser) >> darrow >> doSeq
 def doCatchMatch := leading_parser

src/Lean/Parser/Term.lean

@@ -802,7 +802,6 @@ interpolated string literal) to stderr. It should only be used for debugging.
 @[builtin_term_parser] def assert := leading_parser:leadPrec
   withPosition ("assert! " >> termParser) >> optSemicolon termParser
 
-
 def macroArg       := termParser maxPrec
 def macroDollarArg := leading_parser "$" >> termParser 10
 def macroLastArg   := macroDollarArg <|> macroArg
@@ -823,6 +822,19 @@ Implementation of the `show_term` term elaborator.
 @[builtin_term_parser] def showTermElabImpl :=
   leading_parser:leadPrec "show_term_elab " >> termParser
 
+/-!
+`match_expr` support.
+-/
+
+def matchExprAlt (rhsParser : Parser) := leading_parser "| " >> ppIndent (optional (atomic (ident >> "@")) >> ident >> many binderIdent >> " => " >> rhsParser)
+def matchExprElseAlt (rhsParser : Parser) := leading_parser "| " >> ppIndent (hole >> " => " >> rhsParser)
+def matchExprAlts (rhsParser : Parser) :=
+  leading_parser withPosition $
+    many (ppLine >> checkColGe "irrelevant" >> notFollowedBy (symbol "| " >> " _ ") "irrelevant" >> matchExprAlt rhsParser)
+    >> (ppLine >> checkColGe "irrelevant" >> matchExprElseAlt rhsParser)
+@[builtin_term_parser] def matchExpr := leading_parser:leadPrec
+  "match_expr " >> termParser >> " with" >> ppDedent (matchExprAlts termParser)
+
 end Term
 
 @[builtin_term_parser default+1] def Tactic.quot : Parser := leading_parser
@@ -841,6 +853,7 @@ builtin_initialize
   register_parser_alias matchDiscr
   register_parser_alias bracketedBinder
   register_parser_alias attrKind
+  register_parser_alias optSemicolon
 
 end Parser
 end Lean