feat(compiler): Support pattern aliases

Author: ospencer

compiler/src/parsing/parser.messages

@@ -1247,6 +1247,32 @@ program: EXPORT TYPEID AS WHILE
 ## The known suffix of the stack is as follows:
 ## AS
 ##
+program: LET NUMBER_INT AS WHILE
+##
+## Ends in an error in state: 367.
+##
+## pattern -> pattern AS . id_str [ WHEN THICKARROW RPAREN RBRACK RBRACE PIPE EQUAL EOL COMMA COLON AS ]
+## pattern -> pattern AS . eols id_str [ WHEN THICKARROW RPAREN RBRACK RBRACE PIPE EQUAL EOL COMMA COLON AS ]
+##
+## The known suffix of the stack is as follows:
+## pattern AS
+##
+program: LET NUMBER_INT AS EOL WHILE
+##
+## Ends in an error in state: 369.
+##
+## pattern -> pattern AS eols . id_str [ WHEN THICKARROW RPAREN RBRACK RBRACE PIPE EQUAL EOL COMMA COLON AS ]
+##
+## The known suffix of the stack is as follows:
+## pattern AS eols
+##
+## WARNING: This example involves spurious reductions.
+## This implies that, although the LR(1) items shown above provide an
+## accurate view of the past (what has been recognized so far), they
+## may provide an INCOMPLETE view of the future (what was expected next).
+## In state 3, spurious reduction of production nonempty_list(eol) -> EOL
+## In state 5, spurious reduction of production eols -> nonempty_list(eol)
+##
 
 Expected an identifier to use as an alias.
 

compiler/src/parsing/parser.mly

@@ -60,6 +60,7 @@ module Grain_parsing = struct end
 
 %nonassoc _below_infix
 
+%left AS
 %left PIPEPIPE
 %left AMPAMP
 %left PIPE
@@ -250,7 +251,8 @@ pattern:
   | type_id { Pat.construct ~loc:(to_loc $loc) $1 [] }
   | lbrack rbrack { Pat.list ~loc:(to_loc $loc) [] }
   | lbrack lseparated_nonempty_list(comma, list_item_pat) comma? rbrack { Pat.list ~loc:(to_loc $loc) $2 }
-  | pattern pipe_op opt_eols pattern { Pat.or_ ~loc:(to_loc $loc) $1 $4 }
+  | pattern pipe_op opt_eols pattern %prec PIPE { Pat.or_ ~loc:(to_loc $loc) $1 $4 }
+  | pattern AS opt_eols id_str { Pat.alias ~loc:(to_loc $loc) $1 $4 }
 
 list_item_pat:
   | ELLIPSIS pattern { ListSpread ($2, to_loc $loc) }

compiler/test/suites/pattern_matching.re

@@ -222,6 +222,18 @@ describe("pattern matching", ({test}) => {
     "or_match_3",
     "match ([5]) { [a, _] | [_, a, _] | [a] => true, _ => false }",
   );
+  // Aliases
+  assertSnapshot("alias_match_1", "match (true) { _ as p => p }");
+  assertSnapshot("alias_match_2", "match (true) { a as b => a && b }");
+  assertSnapshot("alias_match_3", "match (true) { true | false as p => p }");
+  assertSnapshot(
+    "alias_match_4",
+    "match (Some(5)) { Some(3 | 4 as a) => a, Some(_) | None => 5, _ => 6 }",
+  );
+  assertSnapshot(
+    "alias_match_5",
+    "match (Some(5)) { Some(3 | 4) as a => a, Some(5) | None as a => a, _ => None }",
+  );
   assertFileRun("mixed_matching", "mixedPatternMatching", "true\n");
   assertWarning(
     "bool_exhaustiveness1",