Initial data commit

Task/Abstract-type/Mercury/abstract-type.mercury

@@ -0,0 +1,26 @@
+:- module eq.
+:- interface.
+
+:- typeclass eq(T) where [
+    pred (T::in) == (T::in) is semidet,
+    pred (T::in) \= (T::in) is semidet
+].
+
+:- pred f(T::in) is semidet <= eq(T).
+
+:- type foo
+    --->    foo(
+                x :: int,
+                str :: string
+            ).
+
+:- instance eq(foo).
+
+:- implementation.
+
+f(X) :- X == X.
+
+:- instance eq(foo) where [
+    A == B :- (A^x = B^x, A^str = B^str),
+    A \= B :- not A == B
+].