lang: parser: Add lookup functions to lexer/parser

This seems to make things work. I'm worried I might have an ordering bug
where we might choose the wrong precedence if we write ambiguous code
somehow, but at least for now, let's commit this and move on. Hopefully
the nonassoc stuff is actually correct.

lang/parser/lexer.nex

@@ -44,6 +44,11 @@
 			lval.str = yylex.Text()
 			return ELVIS
 		}
+/\|\|/		{
+			yylex.pos(lval) // our pos
+			lval.str = yylex.Text()
+			return DEFAULT
+		}
 /=>/		{
 			yylex.pos(lval) // our pos
 			lval.str = yylex.Text()

lang/parser/parser.y

@@ -83,7 +83,7 @@ func init() {
 %token BOOL STRING INTEGER FLOAT
 %token EQUALS DOLLAR
 %token COMMA COLON SEMICOLON
-%token ELVIS ROCKET ARROW DOT
+%token ELVIS DEFAULT ROCKET ARROW DOT
 %token BOOL_IDENTIFIER STR_IDENTIFIER INT_IDENTIFIER FLOAT_IDENTIFIER
 %token MAP_IDENTIFIER STRUCT_IDENTIFIER VARIANT_IDENTIFIER
 %token RES_IDENTIFIER
@@ -105,7 +105,10 @@ func init() {
 %left MULTIPLY DIVIDE
 %right NOT
 //%right EXP	// exponentiation
-%nonassoc IN	// TODO: is %nonassoc correct for this?
+%nonassoc ARROW		// XXX: is %nonassoc correct for this?
+%nonassoc DEFAULT	// XXX: is %nonassoc correct for this?
+%nonassoc OPEN_BRACK	// XXX: is %nonassoc correct for this?
+%nonassoc IN		// XXX: is %nonassoc correct for this?
 
 %error IDENTIFIER STRING OPEN_CURLY IDENTIFIER ROCKET BOOL CLOSE_CURLY: errstrParseExpectingComma
 %error IDENTIFIER STRING OPEN_CURLY IDENTIFIER ROCKET STRING CLOSE_CURLY: errstrParseExpectingComma
@@ -702,6 +705,70 @@ call:
 			},
 		}
 	}
+	// lookup an index in a list or a key in a map
+	// lookup($foo, $key)
+	// `$foo[$key]` // no default specifier
+|	expr OPEN_BRACK expr CLOSE_BRACK
+	{
+		posLast(yylex, yyDollar) // our pos
+		$$.expr = &ast.ExprCall{
+			Name: funcs.LookupFuncName,
+			Args: []interfaces.Expr{
+				$1.expr, // the list or map
+				$3.expr, // the index or key is an expr
+				//$6.expr, // the default
+			},
+		}
+	}
+	// lookup an index in a list or a key in a map with a default
+	// lookup_default($foo, $key, $default)
+	// `$foo[$key] || "default"`
+|	expr OPEN_BRACK expr CLOSE_BRACK DEFAULT expr
+	{
+		posLast(yylex, yyDollar) // our pos
+		$$.expr = &ast.ExprCall{
+			Name: funcs.LookupDefaultFuncName,
+			Args: []interfaces.Expr{
+				$1.expr, // the list or map
+				$3.expr, // the index or key is an expr
+				$6.expr, // the default
+			},
+		}
+	}
+	// lookup a field in a struct
+	// _struct_lookup($foo, "field")
+	// $foo->field
+|	expr ARROW IDENTIFIER
+	{
+		posLast(yylex, yyDollar) // our pos
+		$$.expr = &ast.ExprCall{
+			Name: funcs.StructLookupFuncName,
+			Args: []interfaces.Expr{
+				$1.expr, // the struct
+				&ast.ExprStr{
+					V: $3.str, // the field is always an str
+				},
+				//$5.expr, // the default
+			},
+		}
+	}
+	// lookup a field in a struct with a default
+	// _struct_lookup_optional($foo, "field", "default")
+	// $foo->field || "default"
+|	expr ARROW IDENTIFIER DEFAULT expr
+	{
+		posLast(yylex, yyDollar) // our pos
+		$$.expr = &ast.ExprCall{
+			Name: funcs.StructLookupOptionalFuncName,
+			Args: []interfaces.Expr{
+				$1.expr, // the struct
+				&ast.ExprStr{
+					V: $3.str, // the field is always an str
+				},
+				$5.expr, // the default
+			},
+		}
+	}
 |	expr IN expr
 	{
 		posLast(yylex, yyDollar) // our pos