typelevel-1.2.3: Useful type level operations (type families and related operators).

• type ($$) f a = f a • type () f a = f a • type (&) a f = f a # Documentation type ($$) f a = f a infixr 0 Source #
The $$ operator is just like  one but with even lower precedence level. Unlike value-level , the type-level one has precedence level of infixr 1 in order to be used in function arguments, like edge :: Node  Source a -> Node  Target a -> a. The $$ operator has higher precedence than ->, so the above expression would not be valid when using it.