gforth-tutorial/11-conditionals.fth

\ -1: true
\ 0: false
\ Read further for more info.

\ Conditionals can only be used inside colon definitions.

: abs ( n1 -- +n2 )
  dup 0 < if negate endif ;

\ `if` looks at the topmost cell on the stack. If it
\ contains a non-zero value, it is interpreted as truthy. If
\ it is zero, it is interpreted as falsy. Usually -1 is used
\ for true.

\ `endif` is equivalent to `then`. The tutorial gives the
\ following definition for endif:

( : endif postpone then ; immediate )

: abs ( n1 -- +n2 )
  dup 0 < if negate then ;

\ There is also an if-else-then:

: min ( n1 n2 -- n )
  2dup < if drop else nip then ;

2 3 min .
3 2 min .

\ Exercise: min without using an else branch.

: min ( n1 n2 -- n )
  \ Only let the `if` decide whether or not to swap the
  \ arguments.
  2dup > if swap then
  drop ;