toy-forth-in-picolisp

This toy Forth is implemented using the OOP capabilities of PicoLisp. It is mostly intended as a proof-of-concept, and very little has been done to keep the user from doing stupid errors. In addition, this will probably not teach you much about how to implement real Forth.

I suggest you run it like this:

pil path/to/forth.l +

All predefined words (except pause and bye) are implemented as PicoLisp methods of the +Forth class. When new words are defined, they are also implemented as methods, but on the current +Forth object only, not on the class. This way one may have a few +Forth instances, existing at the same time, differing in the words that are defined.

These are the only (fairly standard) predefined words currently available:

• drop
• dup
• over
• swap
• 1+
• 1-
• +
• *
• <
• >
• i
• .s

In addition you may use pause to temporarily exit out into the PicoLisp REPL, assuming you started this forth.l as recommended. You may then get back into the same +Forth object again doing (resume). The other special word is bye, which exits out into the shell.

To define a new word, you use a sequence of words or numbers starting with a : and ending with a ;, where the first word after the : is the name of the new word. In this implementation you may also create recursive words. A recursive definition of the factorial function can be done like this:

: fac dup 1 > if dup 1- fac * then ;

To use this new fac word to compute 5!, and then display the stack, you just do this: 5 fac .s

There are a few word patterns that involve keywords, and those keywords must be regarded as reserved words. The patterns are:

• : wordname token1 token2 ... ;
• if token1 ... [else token2 ...] then
• do token1 ... loop

Some other Forth implementations doesn't allow if-else-then and do-loop to be used outside word definition blocks, but with this implementation you may do that.

The three patterns mentioned above are implemented as classes (+DefineWord, +IfElseThen, +DoLoop). As subclasses of +WordBlock they are instantiated and executed in similar ways: When the first keyword (:, if, do) is received, an object of the corresponding class is constructed. Subsequent tokens are then fed to this object, until the closing keyword is received. If this object is not part/child of an enclosing pattern object, then the execute> method of the object will be called.

Example: When we, in this Forth REPL, enter the line defining the fac word above, the execute> method of the +IfElseThen will not be called when the then word arrives, since that object is part of another pattern, but the execute> method of the +DefineWord will be called, meaning that the new word will be defined (but not executed).

At the end of the forth.l file there are a couple of references to *EMUENV, and two functions emuForthReplHandler and activateEmuForthRepl. Please just ignore them, as this forth.l currently doesn't work with EmuLisp.

Enjoy!

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PS. Another, quite different, implementation of Forth in PicoLisp can be found here:

http://software-lab.de/forth.l