ppx_overload provides SML style overloading of values and functions. This is not so powerful as the type classes, i.e., the overloaded instances must be resolved locally and you cannot inherit overloading by let-polymorphism. But it is very simple but useful.
Overloaded value must be declared by
external <name> : <type> = "%OVERLOADED". The type should be the least general anti-unifier of values overloaded. Then its instances should be defined in sub-modules of this
external declaration with the same name. Instance types must be less general than the type at the
external declaration. Here is an example of overloaded
module Plus = struct external (+) : 'a -> 'a -> 'a = "%OVERLOADED" module Int = struct let (+) = Pervasives.(+) end module Float = struct let (+) = Pervasives.(+.) end end
The use of an overloaded value is replaced by one of its instances depending of its typing. If there is no match or if there are more than one match, this overload resolution fails:
Plus.(+) 1 2; (* converted to Int.(+) 1 2 *) Plus.(+) 1.2 3.4; (* converted to Float.(+) 1.2 3.4 *)
You cannot derive overloading by let-polymorphism:
let double x = Plus.(+) x x (* error *)
You can extend the overloading pretty easily by module inclusion:
module Plus' = struct include Plus module String = struct let (+) = String.(^) end Plus'.(+) 1 2; (* converted to Int.(+) 1 2 *) Plus'.(+) 1.2 3.4; (* converted to Float.(+) 1.2 3.4 *) Plus'.(+) "x" "y"; (* converted to String.(+) "x" "y" *)
ppx_overload does not work with toplevel.