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Task/Fibonacci-sequence/Mercury/fibonacci-sequence-1.mercury

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+% The following code is derived from the Mercury Tutorial by Ralph Becket.
+% http://www.mercury.csse.unimelb.edu.au/information/papers/book.pdf
+:- module fib.
+
+:- interface.
+:- import_module io.
+:- pred main(io::di, io::uo) is det.
+
+:- implementation.
+:- import_module int.
+
+:- pred fib(int::in, int::out) is det.
+fib(N, X) :-
+    ( if N =< 2
+          then X = 1
+          else fib(N - 1, A), fib(N - 2, B), X = A + B ).
+
+:- func fib(int) = int is det.
+fib(N) = X :- fib(N, X).
+
+main(!IO) :-
+    fib(40, X),
+    write_string("fib(40, ", !IO),
+    write_int(X, !IO),
+    write_string(")\n", !IO),
+
+    write_string("fib(40) = ", !IO),
+    write_int(fib(40), !IO),
+    write_string("\n", !IO).

Task/Fibonacci-sequence/Mercury/fibonacci-sequence-2.mercury

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+:- pred fib_acc(int::in, int::in, int::in, int::in, int::out) is det.
+
+fib_acc(N, Limit, Prev2, Prev1, Res) :-
+    ( N < Limit ->
+        % limit not reached, continue computation.
+        ( N =< 2 ->
+            Res0 = 1
+        ;
+            Res0 = Prev2 + Prev1
+        ),
+        fib_acc(N+1, Limit, Prev1, Res0, Res)
+    ;
+        % Limit reached, return the sum of the two previous results.
+        Res = Prev2 + Prev1
+    ).

Task/Fibonacci-sequence/Mercury/fibonacci-sequence-3.mercury

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+fib_acc(1, 40, 1, 1, Result)

Task/Fibonacci-sequence/Mercury/fibonacci-sequence-4.mercury

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+:- pragma memo(fib/2).
+:- pred fib(int::in, int::out) is det.
+fib(N, X) :-
+    ( if N =< 2
+          then X = 1
+          else fib(N - 1, A), fib(N - 2, B), X = A + B ).