These types are needed to accurately represent C function prototypes, in order to access C library interfaces in Haskell. The Haskell system is not required to represent those types exactly as C does, but the following guarantees are provided concerning a Haskell type CT representing a C type t:

• If a C function prototype has t as an argument or result type, the use of CT in the corresponding position in a foreign declaration permits the Haskell program to access the full range of values encoded by the C type; and conversely, any Haskell value for CT has a valid representation in C.
• sizeOf (undefined :: CT) will yield the same value as sizeof (t) in C.
• alignment (undefined :: CT) matches the alignment constraint enforced by the C implementation for t.
• The members peek and poke of the Storable class map all values of CT to the corresponding value of t and vice versa.
• When an instance of Bounded is defined for CT, the values of minBound and maxBound coincide with t_MIN and t_MAX in C.
• When an instance of Eq or Ord is defined for CT, the predicates defined by the type class implement the same relation as the corresponding predicate in C on t.
• When an instance of Num, Read, Integral, Fractional, Floating, RealFrac, or RealFloat is defined for CT, the arithmetic operations defined by the type class implement the same function as the corresponding arithmetic operations (if available) in C on t.
• When an instance of Bits is defined for CT, the bitwise operation defined by the type class implement the same function as the corresponding bitwise operation in C on t.