Type classes, newtypes, and laws

The idea

A type class describes a capability implemented by types, such as equality or ordering. A constrained function can request that capability without selecting one concrete representation. Instances supply operations, while class laws describe relationships those operations should satisfy. The compiler checks types, but it does not prove that a custom equality relation is reflexive, symmetric, or transitive.

Newtype gives a distinct type around one representation, useful when two quantities should not be interchangeable merely because both are integers. Deriving suitable standard instances can preserve the wrapped representation’s established behavior. Keep unsafe constructors or conversions behind a deliberate boundary when the domain imposes additional invariants. Do not assume a newtype automatically validates range or units at runtime.

newtype UserId = UserId Integer deriving (Eq, Show)
main = do
  print (UserId 3 == UserId 3)
  print (UserId 3 == UserId 4)

True
False

The derived equality compares wrapped values while UserId remains a distinct type from an ordinary Integer.