ShowSourceShow plugin: converts value to a string.
Synthetized attributes' type (both default and for type parameters) is string.
Inherited attributes' type (both default and for type parameters) is unit.
For type declaration type ('a,'b,...) typ = ... it will create transformation function with type
('a -> string) -> ('b -> string) -> ... -> ('a,'b,...) typ -> string
See also: Fmt plugin.