jberryman <brandon.m.simm...@gmail.com> writes: > This may be a dumb question, but why can we not declare a Monad > instance of a type synonym? This question came to me while working > with the State monad recently and feeling that the requirement that we > wrap our functions in the State constructor is a bit... kludgy. >
Because type defines an _alias_. If you define "type Foo = Maybe Int", then everywhere you have a "Foo" the compiler should be able to replace it with "Maybe Int". As such, if you have a custom instance on your type synonym (say a custom Show instance for Foo), then which instance will the compiler use? -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
