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+# An implementation of Dartmouth BASIC (1964)
+#
+
+from ply import *
+import basiclex
+
+tokens = basiclex.tokens
+
+precedence = (
+ ('left', 'PLUS','MINUS'),
+ ('left', 'TIMES','DIVIDE'),
+ ('left', 'POWER'),
+ ('right','UMINUS')
+)
+
+#### A BASIC program is a series of statements. We represent the program as a
+#### dictionary of tuples indexed by line number.
+
+def p_program(p):
+ '''program : program statement
+ | statement'''
+
+ if len(p) == 2 and p[1]:
+ p[0] = { }
+ line,stat = p[1]
+ p[0][line] = stat
+ elif len(p) ==3:
+ p[0] = p[1]
+ if not p[0]: p[0] = { }
+ if p[2]:
+ line,stat = p[2]
+ p[0][line] = stat
+
+#### This catch-all rule is used for any catastrophic errors. In this case,
+#### we simply return nothing
+
+def p_program_error(p):
+ '''program : error'''
+ p[0] = None
+ p.parser.error = 1
+
+#### Format of all BASIC statements.
+
+def p_statement(p):
+ '''statement : INTEGER command NEWLINE'''
+ if isinstance(p[2],str):
+ print p[2],"AT LINE", p[1]
+ p[0] = None
+ p.parser.error = 1
+ else:
+ lineno = int(p[1])
+ p[0] = (lineno,p[2])
+
+#### Interactive statements.
+
+def p_statement_interactive(p):
+ '''statement : RUN NEWLINE
+ | LIST NEWLINE
+ | NEW NEWLINE'''
+ p[0] = (0, (p[1],0))
+
+#### Blank line number
+def p_statement_blank(p):
+ '''statement : INTEGER NEWLINE'''
+ p[0] = (0,('BLANK',int(p[1])))
+
+#### Error handling for malformed statements
+
+def p_statement_bad(p):
+ '''statement : INTEGER error NEWLINE'''
+ print "MALFORMED STATEMENT AT LINE", p[1]
+ p[0] = None
+ p.parser.error = 1
+
+#### Blank line
+
+def p_statement_newline(p):
+ '''statement : NEWLINE'''
+ p[0] = None
+
+#### LET statement
+
+def p_command_let(p):
+ '''command : LET variable EQUALS expr'''
+ p[0] = ('LET',p[2],p[4])
+
+def p_command_let_bad(p):
+ '''command : LET variable EQUALS error'''
+ p[0] = "BAD EXPRESSION IN LET"
+
+#### READ statement
+
+def p_command_read(p):
+ '''command : READ varlist'''
+ p[0] = ('READ',p[2])
+
+def p_command_read_bad(p):
+ '''command : READ error'''
+ p[0] = "MALFORMED VARIABLE LIST IN READ"
+
+#### DATA statement
+
+def p_command_data(p):
+ '''command : DATA numlist'''
+ p[0] = ('DATA',p[2])
+
+def p_command_data_bad(p):
+ '''command : DATA error'''
+ p[0] = "MALFORMED NUMBER LIST IN DATA"
+
+#### PRINT statement
+
+def p_command_print(p):
+ '''command : PRINT plist optend'''
+ p[0] = ('PRINT',p[2],p[3])
+
+def p_command_print_bad(p):
+ '''command : PRINT error'''
+ p[0] = "MALFORMED PRINT STATEMENT"
+
+#### Optional ending on PRINT. Either a comma (,) or semicolon (;)
+
+def p_optend(p):
+ '''optend : COMMA
+ | SEMI
+ |'''
+ if len(p) == 2:
+ p[0] = p[1]
+ else:
+ p[0] = None
+
+#### PRINT statement with no arguments
+
+def p_command_print_empty(p):
+ '''command : PRINT'''
+ p[0] = ('PRINT',[],None)
+
+#### GOTO statement
+
+def p_command_goto(p):
+ '''command : GOTO INTEGER'''
+ p[0] = ('GOTO',int(p[2]))
+
+def p_command_goto_bad(p):
+ '''command : GOTO error'''
+ p[0] = "INVALID LINE NUMBER IN GOTO"
+
+#### IF-THEN statement
+
+def p_command_if(p):
+ '''command : IF relexpr THEN INTEGER'''
+ p[0] = ('IF',p[2],int(p[4]))
+
+def p_command_if_bad(p):
+ '''command : IF error THEN INTEGER'''
+ p[0] = "BAD RELATIONAL EXPRESSION"
+
+def p_command_if_bad2(p):
+ '''command : IF relexpr THEN error'''
+ p[0] = "INVALID LINE NUMBER IN THEN"
+
+#### FOR statement
+
+def p_command_for(p):
+ '''command : FOR ID EQUALS expr TO expr optstep'''
+ p[0] = ('FOR',p[2],p[4],p[6],p[7])
+
+def p_command_for_bad_initial(p):
+ '''command : FOR ID EQUALS error TO expr optstep'''
+ p[0] = "BAD INITIAL VALUE IN FOR STATEMENT"
+
+def p_command_for_bad_final(p):
+ '''command : FOR ID EQUALS expr TO error optstep'''
+ p[0] = "BAD FINAL VALUE IN FOR STATEMENT"
+
+def p_command_for_bad_step(p):
+ '''command : FOR ID EQUALS expr TO expr STEP error'''
+ p[0] = "MALFORMED STEP IN FOR STATEMENT"
+
+#### Optional STEP qualifier on FOR statement
+
+def p_optstep(p):
+ '''optstep : STEP expr
+ | empty'''
+ if len(p) == 3:
+ p[0] = p[2]
+ else:
+ p[0] = None
+
+#### NEXT statement
+
+def p_command_next(p):
+ '''command : NEXT ID'''
+
+ p[0] = ('NEXT',p[2])
+
+def p_command_next_bad(p):
+ '''command : NEXT error'''
+ p[0] = "MALFORMED NEXT"
+
+#### END statement
+
+def p_command_end(p):
+ '''command : END'''
+ p[0] = ('END',)
+
+#### REM statement
+
+def p_command_rem(p):
+ '''command : REM'''
+ p[0] = ('REM',p[1])
+
+#### STOP statement
+
+def p_command_stop(p):
+ '''command : STOP'''
+ p[0] = ('STOP',)
+
+#### DEF statement
+
+def p_command_def(p):
+ '''command : DEF ID LPAREN ID RPAREN EQUALS expr'''
+ p[0] = ('FUNC',p[2],p[4],p[7])
+
+def p_command_def_bad_rhs(p):
+ '''command : DEF ID LPAREN ID RPAREN EQUALS error'''
+ p[0] = "BAD EXPRESSION IN DEF STATEMENT"
+
+def p_command_def_bad_arg(p):
+ '''command : DEF ID LPAREN error RPAREN EQUALS expr'''
+ p[0] = "BAD ARGUMENT IN DEF STATEMENT"
+
+#### GOSUB statement
+
+def p_command_gosub(p):
+ '''command : GOSUB INTEGER'''
+ p[0] = ('GOSUB',int(p[2]))
+
+def p_command_gosub_bad(p):
+ '''command : GOSUB error'''
+ p[0] = "INVALID LINE NUMBER IN GOSUB"
+
+#### RETURN statement
+
+def p_command_return(p):
+ '''command : RETURN'''
+ p[0] = ('RETURN',)
+
+#### DIM statement
+
+def p_command_dim(p):
+ '''command : DIM dimlist'''
+ p[0] = ('DIM',p[2])
+
+def p_command_dim_bad(p):
+ '''command : DIM error'''
+ p[0] = "MALFORMED VARIABLE LIST IN DIM"
+
+#### List of variables supplied to DIM statement
+
+def p_dimlist(p):
+ '''dimlist : dimlist COMMA dimitem
+ | dimitem'''
+ if len(p) == 4:
+ p[0] = p[1]
+ p[0].append(p[3])
+ else:
+ p[0] = [p[1]]
+
+#### DIM items
+
+def p_dimitem_single(p):
+ '''dimitem : ID LPAREN INTEGER RPAREN'''
+ p[0] = (p[1],eval(p[3]),0)
+
+def p_dimitem_double(p):
+ '''dimitem : ID LPAREN INTEGER COMMA INTEGER RPAREN'''
+ p[0] = (p[1],eval(p[3]),eval(p[5]))
+
+#### Arithmetic expressions
+
+def p_expr_binary(p):
+ '''expr : expr PLUS expr
+ | expr MINUS expr
+ | expr TIMES expr
+ | expr DIVIDE expr
+ | expr POWER expr'''
+
+ p[0] = ('BINOP',p[2],p[1],p[3])
+
+def p_expr_number(p):
+ '''expr : INTEGER
+ | FLOAT'''
+ p[0] = ('NUM',eval(p[1]))
+
+def p_expr_variable(p):
+ '''expr : variable'''
+ p[0] = ('VAR',p[1])
+
+def p_expr_group(p):
+ '''expr : LPAREN expr RPAREN'''
+ p[0] = ('GROUP',p[2])
+
+def p_expr_unary(p):
+ '''expr : MINUS expr %prec UMINUS'''
+ p[0] = ('UNARY','-',p[2])
+
+#### Relational expressions
+
+def p_relexpr(p):
+ '''relexpr : expr LT expr
+ | expr LE expr
+ | expr GT expr
+ | expr GE expr
+ | expr EQUALS expr
+ | expr NE expr'''
+ p[0] = ('RELOP',p[2],p[1],p[3])
+
+#### Variables
+
+def p_variable(p):
+ '''variable : ID
+ | ID LPAREN expr RPAREN
+ | ID LPAREN expr COMMA expr RPAREN'''
+ if len(p) == 2:
+ p[0] = (p[1],None,None)
+ elif len(p) == 5:
+ p[0] = (p[1],p[3],None)
+ else:
+ p[0] = (p[1],p[3],p[5])
+
+#### Builds a list of variable targets as a Python list
+
+def p_varlist(p):
+ '''varlist : varlist COMMA variable
+ | variable'''
+ if len(p) > 2:
+ p[0] = p[1]
+ p[0].append(p[3])
+ else:
+ p[0] = [p[1]]
+
+
+#### Builds a list of numbers as a Python list
+
+def p_numlist(p):
+ '''numlist : numlist COMMA number
+ | number'''
+
+ if len(p) > 2:
+ p[0] = p[1]
+ p[0].append(p[3])
+ else:
+ p[0] = [p[1]]
+
+#### A number. May be an integer or a float
+
+def p_number(p):
+ '''number : INTEGER
+ | FLOAT'''
+ p[0] = eval(p[1])
+
+#### A signed number.
+
+def p_number_signed(p):
+ '''number : MINUS INTEGER
+ | MINUS FLOAT'''
+ p[0] = eval("-"+p[2])
+
+#### List of targets for a print statement
+#### Returns a list of tuples (label,expr)
+
+def p_plist(p):
+ '''plist : plist COMMA pitem
+ | pitem'''
+ if len(p) > 3:
+ p[0] = p[1]
+ p[0].append(p[3])
+ else:
+ p[0] = [p[1]]
+
+def p_item_string(p):
+ '''pitem : STRING'''
+ p[0] = (p[1][1:-1],None)
+
+def p_item_string_expr(p):
+ '''pitem : STRING expr'''
+ p[0] = (p[1][1:-1],p[2])
+
+def p_item_expr(p):
+ '''pitem : expr'''
+ p[0] = ("",p[1])
+
+#### Empty
+
+def p_empty(p):
+ '''empty : '''
+
+#### Catastrophic error handler
+def p_error(p):
+ if not p:
+ print "SYNTAX ERROR AT EOF"
+
+bparser = yacc.yacc()
+
+def parse(data):
+ bparser.error = 0
+ p = bparser.parse(data)
+ if bparser.error: return None
+ return p
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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