Re: [Haskell-cafe] GHCi infers a type but refuses it as type signature

On Thu, Jun 25, 2009 at 1:10 PM, David Menendez wrote:

On Thu, Jun 25, 2009 at 12:17 PM, Wei Hu wrote:

...

Could you or anyone else briefly explain how mmtl solves the combinatorical explosion problem? Reading the source code is not very productive for newbies like me. Thanks!

It's a good question, since from what I can tell mmtl does not solve the problem.

Some quick background: If you have M monad transformers and N classes of operations, you normally need M*N instance declarations, i.e., one per transformer per class. Each instance either provides the functionality directly, such as the MonadState instance for StateT, or promotes it from the underlying monad, such as the MonadState instance for ReaderT.

mmtl instead provides 2*M instances. For each transformer, it has one direct instance (such as MonadState (StateT s m)) and one instance which promotes through any transformer (such as MonadState (t (StateT s m))).

The problem is that this limits you to using at most two transformers at a time. For example, the type ErrorT String (ReaderT Int (StateT Int m)) is an instance of MonadError and MonadReader, but not MonadState because it doesn't have the form t (StateT s m).

-- Dave Menendez http://www.eyrie.org/~zednenem/

Very well explained. Thanks.