> Let3>> eval "2 + 7" :: IO Int
> 9
> Let3>> eval "a + b" :: IO Int
> 0
> Let3>> eval "let x = 4 in x * x" :: IO Int
> 16
> Let3>> eval "let x = 4 in x * x + x" :: IO Int
> 20
> Let3>> eval "(let x = 4 in x * x) + x" :: IO Int
> 16
