HE vs ML discrepancy: ML factorial function hangs (doesn't terminate) on non-numeric input

[zariuq]

• I modified the tutorial factorial example to not infinitely recurse on negative input.
• This works for numbers, but ML freezes when given "monkey".

(= (factorial $x)
   (case $x
     ((0 1)
      ($_ 
        (if (> $x 0)
            (* $x (factorial (- $x 1)))
            (Undefined)
        )))
   )
)

• With the following calls:

! (factorial 5)
! (factorial -1)
! (factorial monkey)

HE:

[120]
[(Undefined)]
[(if (> monkey 0) (* monkey (factorial (- monkey 1))) (Undefined))]

ML:

[120]
[(Undefined)]
...