parser

Recursive Descent Parser

This project is a parser created for a madeup language.

• The following keywords are defined:

  ○ int

  ○ string

  ○ set

  ○ print

  ○ println

• There are four operators:

  ○ +

  ○ -

  ○ *

  ○ /

• Grammar rules:

  Prog ::= StmtList

  StmtList ::= { Stmt T_SC } { StmtList }

  Stmt ::= Decl | Set | Print

  Decl ::= T_INT T_ID | T_STRING T_ID

  Set ::= T_SET T_ID Expr

  Print ::= T_PRINT Expr | T_PRINTLN Expr

  Expr ::= Term { (T_PLUS|T_MINUS) Expr }

  Term ::= Primary { (T_STAR|T_SLASH) Term }

  Primary ::= T_ICONST | T_SCONST | T_ID | T_LPAREN Expr T_RPAREN

• Semantic rules:

  1. There are only two types: integer and string.
  2. Variables must be declared before they are used. They are declared with Decl.
  3. There is one single scope for variables.
  4. Variable names must be unique; once a name has been declared, it may not be declared again.
  5. An integer constant is of type integer.
  6. A string constant is of type string.
  7. The type of an identifier is the type assigned when it is declared.
  8. The type of an Expr or a Term is the type of the result of the operation.
  9. Adding two integers results in an integer representing the sum
  10. Subtracting two integers results in an integer representing the difference
  11. Multiplying two integers results in an integer representing the product
  12. Dividing two integers results in an integer representing the quotient
  13. Adding two strings results in a string representing the concatenation
  14. Multiplying an integer by a string results in a string which is the string operand repeated integer times (i.e. 3 * “hi” is “hihihi”)
  15. Dividing two strings results in an a string where the first instance of the denominator that is found in the numerator is removed.
  16. ALL OTHER COMBINATIONS OF OPERATIONS ARE UNDEFINED and are an error
  17. A variable is assigned a value using a Set. The type of the variable being set must match the type of the expression; if it does not this is an error.
  18. An expression can be printed using either “print” or “println”. In both cases, the value of the Expr is printed to the standard output. In the case of “println”, a newline is printed to standard out at the end of the execution of the program