Integer package:numhask

Arbitrary precision integers. In contrast with fixed-size integral types such as Int, the Integer type represents the entire infinite range of integers. Integers are stored in a kind of sign-magnitude form, hence do not expect two's complement form when using bit operations. If the value is small (fit into an Int), IS constructor is used. Otherwise IP and IN constructors are used to store a BigNat representing respectively the positive or the negative value magnitude. Invariant: IP and IN are used iff value doesn't fit in IS

fromInteger is special in two ways:

• numeric integral literals (like "42") are interpreted specifically as "fromInteger (42 :: GHC.Num.Integer)". The prelude version is used as default (or whatever fromInteger is in scope if RebindableSyntax is set).
• The default rules in haskell2010 specify that constraints on fromInteger need to be in a form C v, where v is a Num or a subclass of Num.

So a type synonym such as type FromInteger a = FromIntegral a Integer doesn't work well with type defaulting; hence the need for a separate class.