Extract is:exact

Extract a scalar element from a vector * val: The vector * idx: The index of the scalar within the vector
Extract allows for indexing operations on String or ByteString.
Given a machine state, compute a value out of it
(Extract s lst) pulls the Factor that matches s out of lst, returning a diminished list and, possibly, the extracted Factor.
Extract A [A, B, C] ==> ([B, C], Just A
Extract F [A, B, C] ==> ([A, B, C], Nothing)