Question: programmable calculator (help for a newbie)

[Julián Calderón Almendros]

The scheme of the grammar was more or less :
____________________________________________________________________________
____

%%
input:
        |    input line
;
line:      TK_EOL
        |    expression    TK_EOL
        |    def_funct    TK_EOL
;
expression:    value
                |    TK_OPEN    expression    TK_BINARY_OPERATION
TK_VALUE    TK_CLOSE
                |    TK_OPEN    TK_UNARY_PREFIX_OPERATION    expression
TK_CLOSE
                |    TK_OPEN    expression    TK_UNARY_SUFIX_OPERATION
TK_CLOSE
;
value:    TK_NUM
        |    TK_VAR
        |    TK_EVAL_FUNCTION    TK_FUNCTION    TK_OPEN    lst_args
TK_CLOSE
;
...
;
def_funct:    TK_FUNCT_NAME    TK_OPEN    formal_lst_args    TK_ASIGNATION
expression
;
...
;
%%
____________________________________________________________________________
____
My only problem is: How do I evaluate a function? I take the expression of
the function and I puts it into one string and that into a map<string
name_of_function,string expresion_of_function>. I don't know if the problem
idea is well expressed because my english is very bad.

Thanks on advance.