When you invoke the Forth image, you will see a startup banner printed and nothing else (if you have Gforth installed on your system, try invoking it now, by typing gforth<RET>). Forth is now running its command line interpreter, which is called the Text Interpreter (also known as the Outer Interpreter). (You will learn a lot about the text interpreter as you read through this chapter, for more detail see The Text Interpreter).
Although it's not obvious, Forth is actually waiting for your input. Type a number and press the <RET> key:
Rather than give you a prompt to invite you to input something, the text
interpreter prints a status message after it has processed a line
of input. The status message in this case (“
ok” followed by
carriage-return) indicates that the text interpreter was able to process
all of your input successfully. Now type something illegal:
qwer341<RET> *the terminal*:2: Undefined word >>>qwer341<<< Backtrace: $2A95B42A20 throw $2A95B57FB8 no.extensions
The exact text, other than the “Undefined word” may differ slightly on your system, but the effect is the same; when the text interpreter detects an error, it discards any remaining text on a line, resets certain internal state and prints an error message. For a detailed description of error messages see Error messages.
The text interpreter waits for you to press carriage-return, and then processes your input line. Starting at the beginning of the line, it breaks the line into groups of characters separated by spaces. For each group of characters in turn, it makes two attempts to do something:
If the text interpreter is unable to do either of these things with any
group of characters, it discards the group of characters and the rest of
the line, then prints an error message. If the text interpreter reaches
the end of the line without error, it prints the status message “
followed by carriage-return.
This is the simplest command we can give to the text interpreter:
The text interpreter did everything we asked it to do (nothing) without
an error, so it said that everything is “
ok”. Try a slightly longer
12 dup fred dup<RET> *the terminal*:3: Undefined word 12 dup >>>fred<<< dup Backtrace: $2A95B42A20 throw $2A95B57FB8 no.extensions
When you press the carriage-return key, the text interpreter starts to work its way along the line:
2, it takes the group of characters
12and looks them up in the name dictionary1. There is no match for this group of characters in the name dictionary, so it tries to treat them as a number. It is able to do this successfully, so it puts the number, 12, “on the stack” (whatever that means).
dup. It looks it up in the name dictionary and (you'll have to take my word for this) finds it, and executes the word
dup(whatever that means).
fred. It looks them up in the name dictionary, but can't find them. It tries to treat them as a number, but they don't represent any legal number.
At this point, the text interpreter gives up and prints an error
message. The error message shows exactly how far the text interpreter
got in processing the line. In particular, it shows that the text
interpreter made no attempt to do anything with the final character
dup, even though we have good reason to believe that the
text interpreter would have no problem looking that word up and
executing it a second time.
 We can't tell if it found them or not, but assume for now that it did not