Initial prompt:

  Provide an ANS Forth word that takes a string and character,
  and returns index of first position of character or -1 if not
  found.

Generated code:

  : CHAR-INDEX  ( c-addr u char -- n )
    >R                    \ save character on return stack
    0                     \ initialize index counter
    BEGIN
        DUP 2 PICK U<     \ check if index < string length
    WHILE
        DUP 2 PICK +      \ calculate address of current character
        C@ R@ =           \ compare with target character
        IF
            NIP NIP       \ remove string address and length
            R> DROP       \ clean up return stack
            EXIT          \ return with index
        THEN
        1+                \ increment index
    REPEAT
    DROP 2DROP            \ clean up stack (index, addr, len)
    R> DROP               \ clean up return stack
    -1                    \ return -1 (not found)
  ;

This doesn't work in gforth. I pasted in the error messages and
it gave me a new word which works:

  : CHAR-INDEX  ( c-addr u char -- n )
    >R                    \ save target character
    0                     \ start index at 0
    BEGIN
        DUP 2 PICK U<     \ index < length?
    WHILE
        DUP 3 PICK + C@   \ get char at string[index]
        R@ =              \ equal to target?
        IF
            NIP NIP       \ remove addr and len
            R> DROP       \ clean return stack  
            EXIT          \ return index
        THEN
        1+                \ increment index
    REPEAT
    2DROP                 \ drop addr and len
    DROP                  \ drop index
    R> DROP               \ clean return stack
    -1                    \ not found
  ;

Tests:

  S" Hello World" CHAR H CHAR-INDEX .    \ prints 0
  S" Hello World" CHAR W CHAR-INDEX .    \ prints 6  
  S" Hello World" CHAR Z CHAR-INDEX .    \ prints -1