将单子添加到transformer堆栈的中间

我试着“半提”一个 (ExceptT Error IO Foo) (ExceptT Error (StateT Bar IO) Baz) .

我试过了 lift fmap lift 和 fmap return ,无工作;这里有标准的成语吗?

> import Control.Monad.Except
> import Control.Monad.State
> data Error
> data Foo
> data Bar
> data Baz
> x = undefined :: ExceptT Error IO Foo
> y = undefined :: (ExceptT Error (StateT Bar IO) Baz) -> a

> f = ??? -- This is what I'm trying to find.

> :t y (f x)
y (f x) :: a