Eval monad & EvalT monad transformer
reduce warning
#14 Added Eval.hs
#16 Added SimpleBool Error test
#16 enable display "Something went wrong"
#16 Modify NoRuleApplies message
#16 enable display "Wrong binding"
#16 enable display "Out of index of the Context"
#16 enable display "Undefined Symbol"
Add comment for Error data-type
#16 enable display "Mismatch Type"
#16 enable display "Cannot Type Unify"
#16 rename "Cannot Type Unify"
#16 enable display "Is not Arrow type"
Merge pull request #17 from VoQn/SimpleBool

Added Error data-type for SimpleBool