CODEWITHSUNDEEP
variablesprologinstantiation
lefunction
lefunction

Reputation: 301

Prolog How to get uninstantiated variables to yield output

I have a predicate that receives a binary number in the form of an atom, and returns a list of hexadecimal codes.

?- binaryHex('11111111',X).
X = [f, f].

When I try to go from hexadecimal to binary, I get this error:

?- binaryHex(X,[f,f]).
ERROR: atom_codes/2: Arguments are not sufficiently instantiated

I want both variables to produce the correct output. Here is the code.

hex('0000',0). hex('0001',1). hex('0010',2). hex('0011',3).
hex('0100',4). hex('0101',5). hex('0110',6). hex('0111',7).
hex('1000',8). hex('1001',9). hex('1010',a). hex('1011',b).
hex('1100',c). hex('1101',d). hex('1110',e). hex('1111',f).

binaryHex(X,Y) :-
    atom_codes(X,A),
    binaryList(A,B),
    binaryHex_(B,Y).

binaryList([],[]).
binaryList([A,B,C,D|Ds],[Y|Ys]) :-
    atom_codes(Y,[A,B,C,D]),
    binaryList(Ds,Ys).

binaryHex_([],[]).
binaryHex_([B|Bs],[H|Hs]) :-
    hex(B,H),
    binaryHex_(Bs,Hs).

The variables in the predicates binaryHex_/2 and binaryList/2 work both ways. The program breaks down because of the order of instantiation in the original binaryHex/2: binaryList/2 must come before binaryHex_/2 in order for the original task of binary to hex to work.

I imagine that this problem will be compounded if I convert the hexadecimal list to an atom. What are some strategies to cope with this situation so that as I continue to program I do not run into instantiation errors? Any comments/answers as to when this is not achievable are also encouraged.

Upvotes: 1

Views: 491

Answers (2)

false
false

Reputation: 10142

Your problem is that you are using the wrong representation. Prolog can handle what you want very easily without any extra effort when using lists. That is, lists of characters (= atoms of length 1). Also, it is much more natural to use a . Ah, and 0 alone is an integer, '0' is the corresponding character!

:- set_prolog_flag(double_quotes, chars). % More about double quotes

hexdigit('0') --> "0000".
hexdigit('1') --> "0001".
hexdigit('2') --> "0010".
hexdigit('3') --> "0011".
hexdigit('4') --> "0100".
hexdigit('5') --> "0101".
hexdigit('6') --> "0110".
hexdigit('7') --> "0111".
hexdigit('8') --> "1000".
hexdigit('9') --> "1001".
hexdigit( a ) --> "1010".
hexdigit( b ) --> "1011".
hexdigit( c ) --> "1100".
hexdigit( d ) --> "1101".
hexdigit( e ) --> "1110".
hexdigit( f ) --> "1111".

hexnum([]) -->
   [].
hexnum([D|Ds]) -->
   hexdigit(D),
   hexnum(Ds).

binaryHex(Bin, Hex) :-
   phrase(hexnum(Hex), Bin).

And now some sample usage:

?- binaryHex("11111111",X).
   X = [f,f]
;  false.
?- binaryHex(H,"ff").
   H = ['1', '1', '1', '1', '1', '1', '1', '1'].
?- use_module(double_quotes).
% double_quotes compiled into double_quotes 0.00 sec, 12 clauses
   true.
?- binaryHex(H,"ff").
   H = "11111111".

So you get "bidirectionality" simply by using lists. Coroutining is really overkill here. Worse, your original representation using atoms cannot cope with situations as generating all bitstrings of length 8:

?- length(L,8), binaryHex(L,Hex).
   L = "00000000", Hex = "00"
;  L = "00000001", Hex = "01"
;  L = "00000010", Hex = "02"
;  L = "00000011", Hex = "03"
; ... .

Proving that there are no bitstrings of length 7:

?- length(L,7), binaryHex(L,Hex).
   false.

And proving that bitstrings starting with 10 cannot correspond to a hex number starting with f.

?- binaryHex(['1','0'|L],[f|_]).
   false.

You cannot do all these with atoms and when/2 declarations.

Upvotes: 4

CapelliC
CapelliC

Reputation: 60034

I think that what you're asking for is not achievable - generally - in Prolog. But since I answered a similar question, I would suggest to try

binaryHex(X,Y) :-
    when_2(atom_codes(X,A)),
    when_2(binaryList(A,B)),
    binaryHex_(B,Y).

binaryList([],[]).
binaryList([A,B,C,D|Ds],[Y|Ys]) :-
    when_2(atom_codes(Y,[A,B,C,D])),
    binaryList(Ds,Ys).

:- meta_predicate when_2(0).
when_2(P) :-
    strip_module(P,_,Q),
    Q =.. [_,A0,A1],
    when((ground(A0);ground(A1)), P).

Of course, when/2 must be provided by your system.

Upvotes: 1

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