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4.3 Your first Forth definition

Until now, the examples we've seen have been trivial; we've just been using Forth as a bigger-than-pocket calculator. Also, each calculation we've shown has been a “one-off” – to repeat it we'd need to type it in again1 In this section we'll see how to add new words to Forth's vocabulary.

The easiest way to create a new word is to use a colon definition. We'll define a few and try them out before worrying too much about how they work. Try typing in these examples; be careful to copy the spaces accurately:

     : add-two 2 + . ;
     : greet ." Hello and welcome" ;
     : demo 5 add-two ;

Now try them out:

     greet<RET> Hello and welcome  ok
     greet greet<RET> Hello and welcomeHello and welcome  ok
     4 add-two<RET> 6  ok
     demo<RET> 7  ok
     9 greet demo add-two<RET> Hello and welcome7 11  ok

The first new thing that we've introduced here is the pair of words : and ;. These are used to start and terminate a new definition, respectively. The first word after the : is the name for the new definition.

As you can see from the examples, a definition is built up of words that have already been defined; Forth makes no distinction between definitions that existed when you started the system up, and those that you define yourself.

The examples also introduce the words . (dot), ." (dot-quote) and dup (dewp). Dot takes the value from the top of the stack and displays it. It's like .s except that it only displays the top item of the stack and it is destructive; after it has executed, the number is no longer on the stack. There is always one space printed after the number, and no spaces before it. Dot-quote defines a string (a sequence of characters) that will be printed when the word is executed. The string can contain any printable characters except ". A " has a special function; it is not a Forth word but it acts as a delimiter (the way that delimiters work is described in the next section). Finally, dup duplicates the value at the top of the stack. Try typing 5 dup .s to see what it does.

We already know that the text interpreter searches through the dictionary to locate names. If you've followed the examples earlier, you will already have a definition called add-two. Lets try modifying it by typing in a new definition:

     : add-two dup . ." + 2 =" 2 + . ;<RET> redefined add-two  ok

Forth recognised that we were defining a word that already exists, and printed a message to warn us of that fact. Let's try out the new definition:

     9 add-two<RET> 9 + 2 =11  ok

All that we've actually done here, though, is to create a new definition, with a particular name. The fact that there was already a definition with the same name did not make any difference to the way that the new definition was created (except that Forth printed a warning message). The old definition of add-two still exists (try demo again to see that this is true). Any new definition will use the new definition of add-two, but old definitions continue to use the version that already existed at the time that they were compiled.

Before you go on to the next section, try defining and redefining some words of your own.


[1] That's not quite true. If you press the up-arrow key on your keyboard you should be able to scroll back to any earlier command, edit it and re-enter it.