Postfix with two arguments

Let me start with the second question first as it is more direct. You can see for yourself how these inputs work:

f @ x
x ~f~ y
x // f

f[x]
f[x, y]
f[x]

The first and third only work with a single argument. Similar to but distinct from the first is @@ which is shorthand for Apply, and it allows:

f @@ {x, y}

f[x, y]

Here the Head List is replaced with f. For more clarity see: Scan vs. Map vs. Apply

With the single-argument forms you can use Sequence to insert multiple arguments, but only if the function (f) does not have the Attribute SequenceHold or HoldAllComplete:

f @ Sequence[x, y]
Sequence[x, y] // f

f[x, y]
f[x, y]

And an example of where this will not work:

Rule @ Sequence[x, y]

Rule::argr: Rule called with 1 argument; 2 arguments are expected. >>

Rule[Sequence[x, y]]

If you wish to use a multi-argument function with // application you can make it a parametrized, SubValues, or operator form function, all meaning essentially the same thing. See:

• Define parameterized function
• V10's Operator Forms - what are they good for?

A very simple example:

f[a__][x_] := g[a, x]

Now:

y // f[x]

g[x, y]

x // f[1, 2, 3]

g[1, 2, 3, x]

See the first bulleted link above for more examples and theory.

Related:

• Is it possible to insert arguments into functions when they're used like Function@ or //Function?

• Syntax for prefix mode with multiple arguments using @ shorthand

(Just answering the first question. Others have answered the second question thoroughly.)

It's worth mentioning that you can do

{x,y} // (f @@ #) &

f[x,y]

(No parentheses needed, but I added them for clarity.)

You could even make your own function to do this (slightly) more compactly, especially if you have trouble remembering the @'s and #'s :

ClearAll[pf]
pf[ argList_, f_ ] := f @@ argList

Then you can do what you want using an infix operator:

{x,y} ~pf~ f

f[x,y]

Edit:

Lately I've taken to writing my first example above as

{x,y} // Apply[f] 

or

{x,y} // Apply@f

which I fine somewhat more readable, particularly when there are other pure functions involved. Note that this uses the operator form of Apply.

According to my basic understand of these notations:

For infix notation, the function must be a binary operator, e.g.

g[x_, y_] := x^2 - y^2;

Then

5~g~2

Gives

25-4=21

For postfix notation, the function must take one argument

f[{x_,y_}]:=x^2-y^2

So

{5,2}//f

gives 21