Re: [Haskell-cafe] Why can't we make an instance declaration on a type synonym?

jberryman 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.Miljenovic@gmail.com IvanMiljenovic.wordpress.com