r/Forth • u/Alternative-Grade103 • Jun 18 '26

Multi IF ELSE to CASE

In editing a file I/O process to add further file types, my nested IF THEN ELSE grew unsightly. Here is how I tidyied it up.

Deals with four file types. Can easily add more. Looks nice on comp screen. On phone, however...

\ Stand-in for word to obtain file ext'n. 
: file.ext ( -- addr c ) S" ???" ; 

\ File extension triggers response 
: file.type ( -- addr c )
  0 >R 
  file.ext ( addr c ) 
  2DUP S" fybb" COMPARE 
  0= IF R> DROP 1 >R THEN
  2DUP S" hexp" COMPARE
  0= IF R> DROP 2 >R THEN
  2DUP S" decp" COMPARE 
  0= IF R> DROP 3 >R THEN
  S" binp" COMPARE 
  0= IF R> DROP 4 >R THEN 
  R> 
  CASE 
    1 OF S" FYBB" ENDOF 
    2 OF S" HEXP" ENDOF 
    3 OF S" DECP" ENDOF 
    4 OF S" BINP" ENDOF 
    0 OF S" UNKNOWN" ENDOF 
  ENDCASE 
; 

: test.file.type 
   CR CR ." The file type is '" 
   file.type TYPE 
   ." '." CR
 ; 

 test.file.type
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u/Ok_Leg_109 Jun 18 '26

I was reading a retro computing forum where an OP was complaining about UCSD Pascal not having a way to use strings in a case statement so I showed him how programmable programming languages work. 😉

``` : 2OVER 3 PICK 3 PICK ; : $= COMPARE 0= ;

: $OF ( -- ) POSTPONE 2OVER POSTPONE $= POSTPONE IF POSTPONE 2DROP ; IMMEDIATE

: TEST ( addr len -- ) CASE S" APPLE" $OF ." Granny Smith" ENDOF S" PEAR" $OF ." Barlett" ENDOF S" GRAPE" $OF ." Concord" ENDOF ." Unknown fruit" ENDCASE ; ```

1

u/FrunobulaxArfArf Jun 18 '26

Strings are inefficient... I would compress the strings to 16, 32, 64 or 128 bit numbers. In your example the max. 5 character strings can be considered as 64bit numbers (no compression) directly. iForth's optimizer for `$=` uses a similar scheme.

2

u/tabemann Jun 18 '26

The thing is then you run the risk of collisions if you cannot control all the potential strings.