Parse conditionals

lib/chervil/ast.rb

@@ -4,6 +4,7 @@ module Chervil
   module AST
     require "chervil/ast/application"
     require "chervil/ast/boolean"
+    require "chervil/ast/conditional"
     require "chervil/ast/definition"
     require "chervil/ast/function"
     require "chervil/ast/identifier"

lib/chervil/ast/conditional.rb

@@ -0,0 +1,17 @@
+module Chervil::AST
+  class Conditional
+    attr_reader :predicate
+    attr_reader :true_branch
+    attr_reader :false_branch
+
+    def initialize(predicate, true_branch, false_branch)
+      @predicate = predicate
+      @true_branch = true_branch
+      @false_branch = false_branch
+    end
+
+    def ==(other)
+      @predicate == other.predicate && @true_branch == other.true_branch && @false_branch == other.false_branch
+    end
+  end
+end

lib/chervil/parser.rb

@@ -30,8 +30,12 @@ module Chervil
     def application
       eat(:lparen)
 
-      if @current_token.type == :identifier && @current_token.value == 'define'
-        return definition
+      if @current_token.type == :identifier
+        if @current_token.value == 'define'
+          return definition
+        elsif @current_token.value == 'if'
+          return conditional
+        end
       end
 
       expression = expr
@@ -73,6 +77,15 @@ module Chervil
       end
     end
 
+    def conditional
+      eat(:identifier) # the `if`
+      predicate = expr
+      true_branch = expr
+      false_branch = expr
+      eat(:rparen)
+      AST::Conditional.new(predicate, true_branch, false_branch)
+    end
+
     def identifier
       identifier = eat(:identifier)
       AST::Identifier.new(identifier.value)

spec/parser_spec.rb

@@ -93,5 +93,15 @@ module Chervil
       expect(parse('(+ 1')).to eq(Error.new("Expected rparen but got eof"))
       expect(parse('1)')).to eq(Error.new("Unexpected token rparen: )"))
     end
+
+    it 'parses a conditional' do
+      expect(parse('(if #t 1 0)').first).to eq(
+        AST::Conditional.new(
+          AST::Boolean.new(true),
+          AST::Number.new(1.0),
+          AST::Number.new(0.0),
+        )
+      )
+    end
   end
 end