iterate through alphabet in prolog

Let's say I want to have some rule for comparison similar to

isin(0,_).
isin(N,List) :- member(N,List), write(N), N1 is N-1, isin(N1,List).

but the List will contain alphabet symbols (e.g. [a,b,d,e,h]). How can I send the next element to the iteration? (so N is a symbol, not a number). And if it's not possible, how can I ma开发者_Python百科ke something similar?

Thanks in advance!

---

@julkiewicz is almost there:

isin(a, _).
isin(Char, List) :-
    member(Char, List),
    char_code(Char, Code),
    write(Char),
    Code1 is Code-1,
    char_code(Char1, Code1),
    isin(Char1, List).

Note that the predicate will always succeed on a, just like your original version always succeeds on 0. You can prevent that by changing the base clause to something like

isin(Char, _) :-
    char_code(a, A),
    Char is A-1.

(But this is really an ugly hack.)

---

NOTE: This works on strings, not on symbols as requested by the OP.

Well it seems like characters are interpreted as number lists really. So this works:

?- X = "a".
X = [97].
?- X is "a".
X = 97.
?- X is "a" + 1.
X = 98.

So this is what I'd propose:

isin("a", _).
isin(N, List) :- member(N, List), N1 is N - 1, isin([N1], List).

Haven't written anything in this language for a long time though.