This can be written up in many other forms, plus the possibility of currying deserves to be pointed out:

$ nix repl
nix-repl> (sq: (x: y: sq y + sq x) 2 7) (x: x*x)
53

nix-repl> (sq: (x: y: sq y + sq x)) (x: x*x)     
«lambda @ (string):1:11»

nix-repl> (sq: (x: y: sq y + sq x)) (x: x*x) 2
«lambda @ (string):1:14»

nix-repl> (sq: (x: y: sq y + sq x)) (x: x*x) 2 7
53

nix-repl> (sq: x: y: sq y + sq x) (x: x*x) 2 7   
53