I am trying to make a class with two parameters and a simple get method. But I am getting an error :Some type variables are unbound in this type. So my question is what am I doing wrong?
class basket num_apples num_bananas=
object
val mutable apples = num_apples
val mutable bananas = num_bananas
method get_a= num_apples (*I suppose it has something to do with this value here*)
end ;;
The full error message is:
Error: Some type variables are unbound in this type:
class basket :
'a ->
'b ->
object
val mutable apples : 'a
val mutable bananas : 'b
method get_a : 'a
end
The method get_a has type 'a where 'a is unbound
Where you can see that the type variables it refers to are 'a and 'b. What it means is that it doesn't know what type these arguments have, because they're not used in any way that suggests their actual type. They could be anything, and if the type variables were parameterized on the class type the compiler would happily accept that. In this case it's fairly obvious that the arguments should be ints, however, so just adding a type annotation for that somewhere in the definition should suffice. Here I've added type annotations to the instance variables:
class basket num_apples num_bananas =
object
val mutable apples: int = num_apples
val mutable bananas: int = num_bananas
method get_a = num_apples
end ;;