Add safe navigator

It behaves like Ruby's safe navigator (&.). Importantly, it does not short-circuit like JavaScript's optional chaining (?.).

Closes #34

Author: MatheusRich

spec/e2e/safe_navigation/does_not_short_circuit.lit

@@ -0,0 +1,2 @@
+let a = nil
+a&.b.c # error: Runtime error: Only instances have properties, got Nil.

spec/e2e/safe_navigation/valid.lit

@@ -0,0 +1,18 @@
+type A {
+  fn b do "b"
+  fn b_nil do nil
+  fn b_c do A()
+  fn c do "c"
+}
+
+let a = nil
+debug(a&.b) # expect: nil
+debug(a&.b&.c) # expect: nil
+
+let a_instance = A()
+debug(a_instance&.b()) # expect: "b"
+debug(a_instance&.b_c()&.c()) # expect: "c"
+
+debug(a_instance&.b_nil()) # expect: nil
+debug(a_instance&.b_nil()&.unknown) # expect: nil
+debug(a_instance.b_nil()&.unknown) # expect: nil

spec/lit/scanner_spec.cr

@@ -44,6 +44,7 @@ describe Lit::Scanner do
   it_scans "<=", to_type: token_type(LESS_EQUAL)
   it_scans "|", to_type: token_type(BAR)
   it_scans "||", to_type: token_type(OR)
+  it_scans "&.", to_type: token_type(AMPERSAND_DOT)
   it_scans "&&", to_type: token_type(AND)
   it_scans "|>", to_type: token_type(PIPE_GREATER)
   it_scans "!=", to_type: token_type(BANG_EQUAL)

src/lit/expr.cr

@@ -151,8 +151,8 @@ module Lit
 
     class If < Expr
       getter condition : Expr
-      getter then_branch : Expr::Block
-      getter else_branch : Expr::Block?
+      getter then_branch : Expr
+      getter else_branch : Expr?
 
       def initialize(@condition, @then_branch, @else_branch); end
 

src/lit/parser.cr

@@ -433,6 +433,13 @@ module Lit
           ignore_newlines
           name = consume(TokenType::IDENTIFIER, "I was expecting a property name after '.'.")
           expr = Expr::Get.new(expr, name)
+        elsif match?(TokenType::AMPERSAND_DOT)
+          token = previous
+          ignore_newlines
+          name = consume(TokenType::IDENTIFIER, "I was expecting a property name after '&.'.")
+          # # Desugar a&.b into: if a != nil then a.b else nil
+          condition = Expr::Binary.new(expr, token.with_type(TokenType::BANG_EQUAL), Expr::Literal.new(nil))
+          expr = Expr::If.new(condition, then_branch: Expr::Get.new(expr, name), else_branch: Expr::Literal.new(nil))
         else
           break
         end

src/lit/scanner.cr

@@ -119,6 +119,8 @@ module Lit
       when '&'
         if match?('&')
           add_token(TokenType::AND)
+        elsif match?('.')
+          add_token(TokenType::AMPERSAND_DOT)
         else
           syntax_error("Unexpected character #{c.inspect}")
         end

src/lit/token_type.cr

@@ -2,7 +2,7 @@ module Lit
   enum TokenType
     LEFT_PAREN; RIGHT_PAREN; LEFT_BRACKET; RIGHT_BRACKET; LEFT_BRACE; RIGHT_BRACE
 
-    COMMA; DOT; COLON; NEWLINE
+    COMMA; DOT; COLON; NEWLINE; AMPERSAND_DOT
 
     # Math
     PLUS; MINUS; SLASH; STAR; STAR_STAR; PERCENT

tools/generate_ast

@@ -18,7 +18,7 @@ module AstGenerator
       "Set                 = object : Expr, name : Token, value : Expr",
       "Grouping            = expression : Expr",
       "Literal             = value : Union(String | Int64 | Float64 | Bool | Nil)",
-      "If                  = condition : Expr, then_branch : Expr::Block, else_branch : Expr::Block?",
+      "If                  = condition : Expr, then_branch : Expr, else_branch : Expr?",
       "Import              = path : Token",
       "Logical             = left : Expr, operator : Token, right : Expr",
       "Self                = keyword : Token",