#lang plait
eval(id)        = error!

eval({let1 {x E1} E2}) = eval(E2[eval(E1)/x])

eval(LAM)       = LAM ; assuming LAM is a function expr

eval({E1 E2}) = if eval(E1) == {lam x Ef} then
                        eval(Ef[eval(E2)/x])   
                else error!