Casting from float or int to string will convert the given number into a decimal ascii string representation.
Casting from string to float or int will do the opposite. IE. Read a decimal number from the string and return that.
Casting from string to program will call cast_to_program in the master object and request a program to return. The standard master object will consider the string a file name and compile the program given by that string. It will then put the program in a chache in case you cast the same string to a program again later.
In all other cases, casts are just compiler hints.