$ ghci

GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> load assign-2-tree.hs

Prelude> :load assign-2-tree.hs 
[1 of 1] Compiling Main             ( assign-2-tree.hs, interpreted )
Ok, modules loaded: Main.

*Main> makeBinTree 3
N (N (N L L) (N L L)) (N (N L L) (N L L))

*Main> let t1 = makeBinTree 3

*Main> t1
N (N (N L L) (N L L)) (N (N L L) (N L L))

*Main> size t1
15