I have a function taking Array Number and I have a value arr = [1,2,3] (so an Array Int) which I want to use as argument.
Therefore I need a converter f :: Int -> Number which I then can use in map f arr to get the Array Number.
I found fromNumber on Pursuit, but no fromInt…
Not sure how you went about finding it, but cases like that are perfect for using pursuit’s ability to search for a type signature. Just in case you weren’t aware, you can actually type Int -> Number into the Pursuit search bar, and see the toNumber function as the first hit.
This is sooooooooo cool! Thanks for the tip!
Another questions about Int and Number (I know I should open a new question for it.):
Why is Number not a generalisation of Int? After all toNumberhas the signature Int -> Number and not Int -> Maybe Number. Then I could use Ints everywhere Numbers are expected. (Probably a silly question for people who understand the type system better…)
PureScript uses what’s called a “Hindley-Milner” type system, which allows it to do type inference. But type inference and subtyping don’t play well together. So when you have a value in PureScript, it is always of a single type. You couldn’t have the value 5 have both type Int and type Number. Instead what you see is that you have one type, but that type might implement multiple type classes. So for example, the value "hello" has type String, but also an instance of Semigroup and Monoid and Ord and Eq, etc. which are all classes. You could write a function that takes in just a String, or you could write a function that takes in any Ord, and then you could pass a String into that function.
Haskell, the language that PureScript drew most of its inspiration from, has a class called Num, and both Int and Number would belong to the Num class. This is closest to what you’re looking for, but PureScript doesn’t have the Num class directly. See this post where Harry states the closest PureScript equivalent is the Ring class. So for example, I could write a function
f :: Number -> Number -> Number
f x y = (x + y) * (x - y)
and then I can’t call it with an Int without first calling toNumber. Or I could write the function as
f :: forall a. Ring a => a -> a -> a
f x y = (x + y) * (x - y)
and now I can call f 8.0 5.0 or f 8 5 and both work.
Thanks @ntwilson! I was briefly coming across Rings in Doco and here on Discourse, but aws always thinking “Naa, I don’t need this now.” - However it certainly seems very useful the way you explain it! 
When time allows I will dig a bit deeper into type classes… (They actually soon come up for me in “The Book”.)