Issue inferring type for generic - crystal-lang

Question

I have a c library that I bind to, which has many structs that hold data of different types.
#[Link("foo")]
lib LibFoo
struct IntStruct
data : LibC::Int
end
struct DoubleStruct
data : LibC::Double
end
fun get_int_struct(n : LibC::SizeT) : IntStruct*
fun get_double_struct(n : LibC::SizeT) : DoubleStruct*
end
I have a generic class where I want the type T to be the type of one of those structs, based off of an input array.
class GenericClass(T)
#ptr : Pointer(T)
#obj : T
def initialize(arr : Array)
#ptr = infer_type(arr)
#obj = #ptr.value
end
end
Where infer_type looks like:
def infer_type(data : Array(Int32))
return LibFoo.get_int_struct(data.size)
end
def infer_type(data : Array(Float64))
return LibFoo.get_double_struct(data.size)
end
The code works fine when I explicitly set the type...
GenericClass(LibFoo::IntStruct).new [1, 2, 3, 4, 5]
...but I would like to infer the type.

Answer 1

Maybe providing a custom generalized new method works for you?
struct A
end
struct B
end
class Generic(T)
def self.new(items : Array(A)) forall A
new(fetch_struct(items))
end
private def initialize(#struct : T)
end
private def self.fetch_struct(items : Array(Int32))
A.new
end
private def self.fetch_struct(items : Array(Float64))
B.new
end
end
p Generic.new([1])
p Generic.new([1.0])
https://carc.in/#/r/7s6d