Why explicitly writing the type `Product Int` works for any `Num` type?

In particular, this expression compiles:

x = 4 :: Product Int

Why does the literal 4 can be Product Int, even though it’s type is Num a => a ?

>Solution :

4 is an expression with a type annotation. The unannotated expression has type Num a => a, but the annotation forces the type to be Product Int.

The annotation is legal because Product a has a Num instance whenever a has a Num instance (which Int has):

> :info Product
newtype Product a = Product {getProduct :: a}
[...]
instance Num a => Num (Product a)
[...]
> Product 3 + Product 5
Product {getProduct = 8}

(which is to say, Product Int is a subtype of Num a => a).