solnic/coercible — pull request #4 by handiwiguna (2013-03-22 18:58:10+00:00)
Title: Raise an exception if the coercion not supported

coercer[String].to_integer('a')
Coercible::UnsupportedCoercion: Coercible::Coercer::String#to_integer doesn't know how to coerce "a"