diff --git a/spec/zodiac/parser_spec.rb b/spec/zodiac/parser_spec.rb
index 58885a3..8e466c3 100644
--- a/spec/zodiac/parser_spec.rb
+++ b/spec/zodiac/parser_spec.rb
@@ -15,5 +15,19 @@
         expect(actual).to eq(expected)
       end
     end
+
+    context 'when simple expression' do
+      xit 'returns simple expression' do
+        parser = described_class.new('1 + 2')
+
+        actual = parser.parse
+        expected = {
+          kind: 'PROGRAM',
+          cmp_stmts: []
+        }
+
+        expect(actual).to eq(expected)
+      end
+    end
   end
 end
diff --git a/src/zodiac/parser.rb b/src/zodiac/parser.rb
index 2100957..e19f3ec 100644
--- a/src/zodiac/parser.rb
+++ b/src/zodiac/parser.rb
@@ -4,153 +4,6 @@
 
 module Zodiac
   # Base parsing class for the Zodiac language.
-
-  # Here is the syntax of Ruby in pseudo BNF. For more detail, see parse.y in Ruby distribution.
-
-  # ARG		: LHS `=' ARG
-  # 		| LHS OP_ASGN ARG
-  # 		| ARG `..' ARG
-  # 		| ARG `...' ARG
-  # 		| ARG `+' ARG
-  # 		| ARG `-' ARG
-  # 		| ARG `*' ARG
-  # 		| ARG `/' ARG
-  # 		| ARG `%' ARG
-  # 		| ARG `**' ARG
-  # 		| `+' ARG
-  # 		| `-' ARG
-  # 		| ARG `|' ARG
-  # 		| ARG `^' ARG
-  # 		| ARG `&' ARG
-  # 		| ARG `<=>' ARG
-  # 		| ARG `>' ARG
-  # 		| ARG `>=' ARG
-  # 		| ARG `<' ARG
-  # 		| ARG `<=' ARG
-  # 		| ARG `==' ARG
-  # 		| ARG `===' ARG
-  # 		| ARG `!=' ARG
-  # 		| ARG `=~' ARG
-  # 		| ARG `!~' ARG
-  # 		| `!' ARG
-  # 		| `~' ARG
-  # 		| ARG `<<' ARG
-  # 		| ARG `>>' ARG
-  # 		| ARG `&&' ARG
-  # 		| ARG `||' ARG
-  # 		| defined? ARG
-  # 		| PRIMARY
-
-  # PRIMARY		: `(' COMPSTMT `)'
-  # 		| LITERAL
-  # 		| VARIABLE
-  # 		| PRIMARY `::' IDENTIFIER
-  # 		| `::' IDENTIFIER
-  # 		| PRIMARY `[' [ARGS] `]'
-  # 		| `[' [ARGS [`,']] `]'
-  # 		| `{' [(ARGS|ASSOCS) [`,']] `}'
-  # 		| return [`(' [CALL_ARGS] `)']
-  # 		| yield [`(' [CALL_ARGS] `)']
-  # 		| defined? `(' ARG `)'
-  #                 | FUNCTION
-  # 		| FUNCTION `{' [`|' [BLOCK_VAR] `|'] COMPSTMT `}'
-  # 		| if EXPR THEN
-  # 		  COMPSTMT
-  # 		  (elsif EXPR THEN COMPSTMT)*
-  # 		  [else COMPSTMT]
-  # 		  end
-  # 		| unless EXPR THEN
-  # 		  COMPSTMT
-  # 		  [else COMPSTMT]
-  # 		  end
-  # 		| while EXPR DO COMPSTMT end
-  # 		| until EXPR DO COMPSTMT end
-  # 		| case COMPSTMT
-  # 		  (when WHEN_ARGS THEN COMPSTMT)+
-  # 		  [else COMPSTMT]
-  # 		  end
-  # 		| for BLOCK_VAR in EXPR DO
-  # 		  COMPSTMT
-  # 		  end
-  # 		| begin
-  # 		  COMPSTMT
-  # 		  [rescue [ARGS] DO COMPSTMT]+
-  # 		  [else COMPSTMT]
-  # 		  [ensure COMPSTMT]
-  # 		  end
-  # 		| class IDENTIFIER [`<' IDENTIFIER]
-  # 		  COMPSTMT
-  # 		  end
-  # 		| module IDENTIFIER
-  # 		  COMPSTMT
-  # 		  end
-  # 		| def FNAME ARGDECL
-  # 		  COMPSTMT
-  # 		  end
-  # 		| def SINGLETON (`.'|`::') FNAME ARGDECL
-  # 		  COMPSTMT
-  # 		  end
-
-  # WHEN_ARGS	: ARGS [`,' `*' ARG]
-  # 		| `*' ARG
-
-  # THEN		: TERM
-  # 		| then
-  # 		| TERM then
-
-  # DO		: TERM
-  # 		| do
-  # 		| TERM do
-
-  # BLOCK_VAR	: LHS
-  # 		| MLHS
-
-  # MLHS		: MLHS_ITEM `,' [MLHS_ITEM (`,' MLHS_ITEM)*] [`*' [LHS]]
-  #                 | `*' LHS
-
-  # MLHS_ITEM	: LHS
-  # 		| '(' MLHS ')'
-
-  # LHS		: VARIABLE
-  # 		| PRIMARY `[' [ARGS] `]'
-  # 		| PRIMARY `.' IDENTIFIER
-
-  # MRHS		: ARGS [`,' `*' ARG]
-  # 		| `*' ARG
-
-  # CALL_ARGS	: ARGS
-  # 		| ARGS [`,' ASSOCS] [`,' `*' ARG] [`,' `&' ARG]
-  # 		| ASSOCS [`,' `*' ARG] [`,' `&' ARG]
-  # 		| `*' ARG [`,' `&' ARG]
-  # 		| `&' ARG
-  # 		| COMMAND
-
-  # ARGS 		: ARG (`,' ARG)*
-
-  # ARGDECL		: `(' ARGLIST `)'
-  # 		| ARGLIST TERM
-
-  # ARGLIST		: IDENTIFIER(`,'IDENTIFIER)*[`,'`*'[IDENTIFIER]][`,'`&'IDENTIFIER]
-  # 		| `*'IDENTIFIER[`,'`&'IDENTIFIER]
-  # 		| [`&'IDENTIFIER]
-
-  # SINGLETON	: VARIABLE
-  # 		| `(' EXPR `)'
-
-  # ASSOCS		: ASSOC (`,' ASSOC)*
-
-  # ASSOC		: ARG `=>' ARG
-
-  # VARIABLE	: VARNAME
-  # 		| nil
-  # 		| self
-
-  # LITERAL		: numeric
-  # 		| SYMBOL
-  # 		| STRING
-  # 		| STRING2
-  # 		| HERE_DOC
-  # 		| REGEXP
   class Parser
     def initialize(raw_string)
       @raw_string = raw_string
@@ -178,6 +31,13 @@ def parse_program
 
     # COMPSTMT	: STMT (TERM EXPR)* [TERM]
     def parse_compstmt
+      parse_stmt
+
+      while @cur_index < @tokens.length
+        parse_term
+        parse_expr
+      end
+
       { kind: 'COMPSTMT', value: nil }
     end
 
@@ -246,5 +106,207 @@ def parse_command
     def parse_function
       { kind: 'FUNCTION', value: nil }
     end
+
+    # THEN		: TERM
+    # 		| then
+    # 		| TERM then
+    def parse_then
+      { kind: 'THEN', value: nil }
+    end
+
+    # DO		: TERM
+    # 		| do
+    # 		| TERM do
+    def parse_do
+      { kind: 'DO', value: nil }
+    end
+
+    # BLOCK_VAR	: LHS
+    # 		| MLHS
+    def parse_block_var
+      { kind: 'BLOCK_VAR', value: nil }
+    end
+
+    # MLHS		: MLHS_ITEM `,' [MLHS_ITEM (`,' MLHS_ITEM)*] [`*' [LHS]]
+    #                 | `*' LHS
+    def parse_mlhs
+      { kind: 'MLHS', value: nil }
+    end
+
+    # ARG		: LHS `=' ARG
+    # 		| LHS OP_ASGN ARG
+    # 		| ARG `..' ARG
+    # 		| ARG `...' ARG
+    # 		| ARG `+' ARG
+    # 		| ARG `-' ARG
+    # 		| ARG `*' ARG
+    # 		| ARG `/' ARG
+    # 		| ARG `%' ARG
+    # 		| ARG `**' ARG
+    # 		| `+' ARG
+    # 		| `-' ARG
+    # 		| ARG `|' ARG
+    # 		| ARG `^' ARG
+    # 		| ARG `&' ARG
+    # 		| ARG `<=>' ARG
+    # 		| ARG `>' ARG
+    # 		| ARG `>=' ARG
+    # 		| ARG `<' ARG
+    # 		| ARG `<=' ARG
+    # 		| ARG `==' ARG
+    # 		| ARG `===' ARG
+    # 		| ARG `!=' ARG
+    # 		| ARG `=~' ARG
+    # 		| ARG `!~' ARG
+    # 		| `!' ARG
+    # 		| `~' ARG
+    # 		| ARG `<<' ARG
+    # 		| ARG `>>' ARG
+    # 		| ARG `&&' ARG
+    # 		| ARG `||' ARG
+    # 		| defined? ARG
+    # 		| PRIMARY
+    def parse_arg
+      { kind: 'ARG', value: nil }
+    end
+
+    # PRIMARY		: `(' COMPSTMT `)'
+    # 		| LITERAL
+    # 		| VARIABLE
+    # 		| PRIMARY `::' IDENTIFIER
+    # 		| `::' IDENTIFIER
+    # 		| PRIMARY `[' [ARGS] `]'
+    # 		| `[' [ARGS [`,']] `]'
+    # 		| `{' [(ARGS|ASSOCS) [`,']] `}'
+    # 		| return [`(' [CALL_ARGS] `)']
+    # 		| yield [`(' [CALL_ARGS] `)']
+    # 		| defined? `(' ARG `)'
+    #                 | FUNCTION
+    # 		| FUNCTION `{' [`|' [BLOCK_VAR] `|'] COMPSTMT `}'
+    # 		| if EXPR THEN
+    # 		  COMPSTMT
+    # 		  (elsif EXPR THEN COMPSTMT)*
+    # 		  [else COMPSTMT]
+    # 		  end
+    # 		| unless EXPR THEN
+    # 		  COMPSTMT
+    # 		  [else COMPSTMT]
+    # 		  end
+    # 		| while EXPR DO COMPSTMT end
+    # 		| until EXPR DO COMPSTMT end
+    # 		| case COMPSTMT
+    # 		  (when WHEN_ARGS THEN COMPSTMT)+
+    # 		  [else COMPSTMT]
+    # 		  end
+    # 		| for BLOCK_VAR in EXPR DO
+    # 		  COMPSTMT
+    # 		  end
+    # 		| begin
+    # 		  COMPSTMT
+    # 		  [rescue [ARGS] DO COMPSTMT]+
+    # 		  [else COMPSTMT]
+    # 		  [ensure COMPSTMT]
+    # 		  end
+    # 		| class IDENTIFIER [`<' IDENTIFIER]
+    # 		  COMPSTMT
+    # 		  end
+    # 		| module IDENTIFIER
+    # 		  COMPSTMT
+    # 		  end
+    # 		| def FNAME ARGDECL
+    # 		  COMPSTMT
+    # 		  end
+    # 		| def SINGLETON (`.'|`::') FNAME ARGDECL
+    # 		  COMPSTMT
+    # 		  end
+    def parse_primary
+      { kind: 'PRIMARY', value: nil }
+    end
+
+    # WHEN_ARGS	: ARGS [`,' `*' ARG]
+    # 		| `*' ARG
+    def parse_when_args
+      { kind: 'WHEN_ARGS', value: nil }
+    end
+
+    # MLHS_ITEM	: LHS
+    # 		| '(' MLHS ')'
+    def parse_mlhs_item
+      { kind: 'MLHS_ITEM', value: nil }
+    end
+
+    # LHS		: VARIABLE
+    # 		| PRIMARY `[' [ARGS] `]'
+    # 		| PRIMARY `.' IDENTIFIER
+    def parse_lhs
+      { kind: 'LHS', value: nil }
+    end
+
+    # MRHS		: ARGS [`,' `*' ARG]
+    # 		| `*' ARG
+    def parse_mrhs
+      { kind: 'MRHS', value: nil }
+    end
+
+    # CALL_ARGS	: ARGS
+    # 		| ARGS [`,' ASSOCS] [`,' `*' ARG] [`,' `&' ARG]
+    # 		| ASSOCS [`,' `*' ARG] [`,' `&' ARG]
+    # 		| `*' ARG [`,' `&' ARG]
+    # 		| `&' ARG
+    # 		| COMMAND
+    def parse_call_args
+      { kind: 'CALL_ARGS', value: nil }
+    end
+
+    # ARGS 		: ARG (`,' ARG)*
+    def parse_args
+      { kind: 'ARGS', value: nil }
+    end
+
+    # ARGDECL		: `(' ARGLIST `)'
+    # 		| ARGLIST TERM
+    def parse_argdecl
+      { kind: 'ARGDECL', value: nil }
+    end
+
+    # ARGLIST		: IDENTIFIER(`,'IDENTIFIER)*[`,'`*'[IDENTIFIER]][`,'`&'IDENTIFIER]
+    # 		| `*'IDENTIFIER[`,'`&'IDENTIFIER]
+    # 		| [`&'IDENTIFIER]
+    def parse_arglist
+      { kind: 'ARGLIST', value: nil }
+    end
+
+    # SINGLETON	: VARIABLE
+    # 		| `(' EXPR `)'
+    def parse_singleton
+      { kind: 'SINGLETON', value: nil }
+    end
+
+    # ASSOCS		: ASSOC (`,' ASSOC)*
+    def parse_assocs
+      { kind: 'ASSOCS', value: nil }
+    end
+
+    # ASSOC		: ARG `=>' ARG
+    def parse_assoc
+      { kind: 'ASSOC', value: nil }
+    end
+
+    # VARIABLE	: VARNAME
+    # 		| nil
+    # 		| self
+    def parse_variable
+      { kind: 'VARIABLE', value: nil }
+    end
+
+    # LITERAL		: numeric
+    # 		| SYMBOL
+    # 		| STRING
+    # 		| STRING2
+    # 		| HERE_DOC
+    # 		| REGEXP
+    def parse_literal
+      { kind: 'LITERAL', value: nil }
+    end
   end
 end