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These are the most powerful and most
used forms of data structures in Forth, and are so called because they
begin with a colon “:”.
Colon definition allow the creation of
new Forth words based on previously defined words. They can be of any length,
although carriage return must be pressed before a particular section.
The general format is:
: new-word word1 word2
... wordn ;
all colon definition end with a
semicolon.
If a word used in a colon definition has
not been previously defined, then an error will result.
The new-word is executed simply by
typing its name and pressing ENTER. e.g. suppose we wish to define a new-word to
calculate the square of a given number, we could do this by:
: SQUARE <CR>
DECIMAL CR ." THE SQUARE OF " <CR>
DUP . ." IS " DUP * . ; <CR>
Where we have defined a new word called SQUARE
which will be called by
SQUARE <CR>
e.g.
9 SQUARE <CR>
will result
in
THE SQUARE OF 9 IS 81
If we follow the operations of the word
we will see the changes in the stack
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TOS
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Operation
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Result
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Emtpy
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9
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9 SQUARE
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9
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CR
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Carriage return
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9
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."
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THE SQUARE OF
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9
, 9
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DUP
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9
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.
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9
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9
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."
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IS
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9
, 9
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DUP
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81
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*
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Empty
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.
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81
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We are now going to discuss control structures. It must be remembered that the
control structures can only be incorporated in colon definition, or an error
will result.
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