为什么无法推断泛型？

Question

I'm just getting started with Nim, and have a problem with generics.

Here is a simplified example that replicates the issue I'm having. Start with a simple object type that has a generic type T , and make an instance of it:

type Foo[T] = object
    bar: T

var my_foo = Foo[string](bar: "foobar")

It works fine if I create a proc with a generic type T , and a return value of T and then return the bar field of the object, which is type T :

# This works!
proc test[T](f: Foo, n: int): T = 
    f.bar

echo test(my_foo, 1)

However, if instead I want to return a sequence of multiple copies of T that come from the Foo.bar field which is type T , like so:

# This fails!
proc test2[T](f: Foo, n: int): seq[T] =
    var s: T = f.bar
    var r: seq[T] = @[] 
    for x in 1..n:
        r.add(s)
    result = r 

echo test2(my_foo, 3)

then I get the following error:

Error: cannot instantiate: 'T'

Could anyone please offer any insight as to why this is and how to correctly do this?

Answer 1

Not sure what the internal reasons for this are, but the type inference in Nim may be too local for this. I'd consider it good style to explicitly state the instantiation of Foo anyway, by writing Foo[T] instead, like this:

proc test2[T](f: Foo[T], n: int): seq[T] =