An expression is a sequence of variables, constants, operators, functions, and parentheses following algebraic rules that calculates a value. Expressions and their intermediate results must be representable as character strings of less than 256 characters.

Every value (in variables, constants, or the result of expressions) has a data type. As **form** calculates an expression, it propagates or converts the data type of values as appropriate. For example, **"5" + 10** evaluates to 15 of data type real. The string **"5"** is converted to real so it can be added to **10**.