CODEWITHSUNDEEP
prologclpfdcollatz
Avrohom Yisroel
Avrohom Yisroel

Reputation: 9472

"Syntax error: Operator expected" with CLPFD Prolog

I'm reading The Power Of Prolog, and have got as far as the section on the Collatz sequence.

I copied and pasted the code from there, adding the appropriate use_module line before...

use_module(library(clpfd)).

hailstone(N, N).
hailstone(N0, N) :-
   N0 #= 2*N1,    % Error
   N0 #> 1,       % ensures termination
   hailstone(N1, N).
hailstone(N0, N) :-
   N0 #= 2*_ + 1, % Error
   N0 #> 1,       % ensures termination
   N1 #= 3*N0 + 1,
   hailstone(N1, N).

However, when I try to compile this, I get an error "hailstone.pl:nn:10: Syntax error: Operator expected" on the two lines commented above.

Anyone any idea what the error is, and why I get it? As you can see, I'm including the CLPFD module, so it should recognise #= as an operator.

I tried using library(clpfd). instead of the first line, but it didn't make any difference.

I'm using SWI-Prolog in case it makes any difference. Thanks.

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