Comment by biscarch

All functions in Haskell are single-argument functions. Multiple arguments are syntactic sugar. The syntax sugar can make it confusing to talk about but if you define an `add` function that takes two arguments x and y:

    add x y = x + y

it's sugar for multiple single-argument functions.

    add = \x -> \y -> x + y

When examining it this way, the `g` function above would translate to

    g = \(x, y) -> 2 * x + y

Which is a function taking a single tuple argument. It is still "default curried" but the argument being passed in is a single argument rather than multiple so we don't expand it to multiple single-arg functions. Perhaps it is more illustrative to show the definition as the single argument it is rather than using haskell's destructuring to pull x and y out of the tuple.

    g tuple = 2 * (fst tuple) + (snd tuple)

and a ghci session for completeness:

    Prelude> let g (x, y) = 2 * x + y
    Prelude> g (1,2)
    4
    Prelude> let y tuple = 2 * (fst tuple) + (snd tuple)
    Prelude> y (1,2)
    4

So when we say that Haskell functions are "curried by default", what we're referring to is roughly the underlying single-argument nature of haskell functions.