[Chicken-hackers] Clean versus pure in types.db

[Peter Bex]

Hi all,

I was looking through types.db and noticed that some procedures
were marked "clean" and others "pure".  If I understand correctly,
the difference is that "clean" procedures may still have side-effects
but will not mutate their input arguments (so pure is a more strict
notion which implies everything clean implies, plus more)

If that's correct, why are procedures like "length" and "list-tail"
marked clean but not pure?

(length (#(procedure #:clean #:enforce) length (list) fixnum) ; may loop
        ((null) '0)
        ((list) (##core#inline "C_u_i_length" #(1))))

(##sys#length (#(procedure #:clean #:enforce) ##sys#length (list) fixnum)
              ((null) '0)
              ((list) (##core#inline "C_u_i_length" #(1))))

(list-tail (forall (a) (#(procedure #:clean #:enforce) list-tail ((list-of a) 
fixnum) (list-of a))))

Now, these are special-cased so maybe that's the reason?  However,
append, reverse and mem* are not special-cased and AFAICT should be
pure but are all only marked "clean".

Cheers,
Peter
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