Type anonymity
Closures succinctly capture variables from enclosing scopes. Does this have any consequences? It surely does. Observe how using a closure as a function parameter requires generics, which is necessary because of how they are defined:
When a closure is defined, the compiler implicitly creates a new
anonymous structure to store the captured variables inside, meanwhile
implementing the functionality via one of the traits
: Fn
, FnMut
, or
FnOnce
for this unknown type. This type is assigned to the variable which
is stored until calling.
Since this new type is of unknown type, any usage in a function will require
generics. However, an unbounded type parameter <T>
would still be ambiguous
and not be allowed. Thus, bounding by one of the traits
: Fn
, FnMut
, or
FnOnce
(which it implements) is sufficient to specify its type.
See also:
A thorough analysis, Fn
, FnMut
,
and FnOnce