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It is about space, not time. What you describe happens interactively. At that time, performance does not matter much, after compiling, it does. At compilation time (the word : is one of the ways to switch the system to compilation mode), Forth doesn't execute words and numbers if finds, but it adds their addresses (in some form, depending on the implementation) to the generated code. When running the code later, the runtime simply fetches each of these function addresses and 'calls' them (that's what makes Forth systems fast without the need for any convoluted compiler technology. Depending on the implementation, that 'call' may be an actual call in the CPU, but it typically is just another 'grab the addresses found there in sequence and execute them' and yes, it can't be turtles all the way down, but Forth gets awfully close) Now, the question is: how do you compile "push this constant on the stack"? Forth does it by compiling the word called LITERAL, followed by the constant. So, compiling a function called 0 or 1 adds a function address to the compiler output, but compiling a literal constant adds the address of the function called LITERAL and that constant. For constants used more than a few (where the exact limit depends on he particular Forth implementation) times, the extra space needed for that function gets more than compensated by the gain. In typical Forth systems, -1, 0, 1, and 2 already are space savers before the user types his first character. That's why they are predefined. If you use another constant often, you can easily define it. Many Forth systems even have a function called CONSTANT for it, but you can define it yourself, if it is absent. For those wondering how the system knows that that constant it compiled isn't the address of a function to call: it doesn't. Instead, the function called LITERAL, when called, hooks into the runtime, uses the 'current instruction pointer' to read the value to push on he stack, and then increases that pointer to point past the constant. When LITERAL returns, the runtime just reads what it thinks is the next pointer and calls it. |