feat(compiler): Support pattern aliases (grain-lang#1174)

* feat(compiler): Support pattern aliases

* snapshots

* Convert snapshot test to runnable test

compiler/src/parsing/parser.messages

@@ -1245,6 +1245,32 @@ 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) }