USING IF Statement with FORTH produces “Interpreting a compile-only word”

Question

I'm reading Starting Forth: 4. Decisions, Decisions... . I can run 42 42 = .

42 42 =  ok
42 42 = . -1  ok

Predictably, I get -1 which is two's compliment for true . However, if I push a 42 on the stack, and I run

42 .s
42 = IF ." foobar " THEN ; 

I would expect foobar to be outputted and it's not. Instead I get

    42 .s <1> 42  ok
    42 = IF ." foobar " THEN ;  
:2: Interpreting a compile-only word
    42 = >>>IF<<< ." foobar " THEN ; 
Backtrace:
$7F7539250B30 throw 

What's going on here?

Answer 1

I believe these must be compiled into words, for whatever reasons expressions aren't primitives. I believe this is referenced in the book with,

Notice: an IF…THEN statement must be contained within a definition. You can't just enter these words in “calculator style.”

So it would look like this,

: mycond 42 = IF ." foobar " THEN ;   ok
42 .s <1> 42  ok
mycond foobar  ok
42 mycond foobar  ok

This is again in the gforth docs on Conditional execution

In Forth you can use control structures only inside colon definitions. An if-structure looks like this:

Answer 2

Exactly structured words are compiled word usage only for loops / if / while...until & so on....

Use Gforth documentation only. This is the best one. I mean to use Gforth ... else you may have difficulties with bad examples or other Forth interpreters specific words not included in ANSI Forth nor gnuforth.

Answer 3

The FORTH standard says that control structures (IF, ELSE, THEN; DO, LOOP; and so on) are for compiled use only, interactive use is an "ambiguous situation". Some implementations allow interactive use, but it is far from universal.