[Haskell-beginners] type inference: how ghc chooses e.g. between Float and Double?

[TP]

Hi all,


In ghci, let us type:

> data Worker x y = Worker x y deriving Show
> let a = Worker 2 3.4
> :t a
a :: Worker Integer Double

According to which procedure does ghci chooses Integer for 2, and Double for 
3.4? I believed that in such a case I was compelled to indicate a type 
signature explicitly, for example:

> let a = Worker (2::Int) (3.4::Float)
> :t a
a :: Worker Int Float

I knew that numbers in Haskell are polymorphic constants instances of the 
Num typeclass:

> :t 2
2 :: Num a => a

But what is happening exactly when he chooses Integer and Double above?

Thanks in advance,

TP