Function Expr constructor with currying
Parse closure
Define Expression
Parse define
eval definition expression
eval to be Monadic
added ErrorT to evaluator
added ReaderT to evaluator
added StateT to evaluator
added WriterT to evaluator
Merge branch 'parse-expression'

* parse-expression:
  added WriterT to evaluator
  added StateT to evaluator
  added ReaderT to evaluator
  added ErrorT to evaluator
  eval to be Monadic
  eval definition expression
  Parse define
  Define Expression
  Parse closure
  Function Expr constructor with currying