[Haskell-beginners] monad transformers
Brandon S. Allbery KF8NH
allbery at ece.cmu.edu
Fri Aug 14 21:05:14 EDT 2009
On Aug 14, 2009, at 20:55 , Michael P Mossey wrote:
> In Martin Grabmuller's tutorial "Monad Transformers Step by Step",
> found here
> he gives an example of composing ErrorT, StateT, ReaderT, and
> WriterT. Early in the paper, where he composes just ErrorT and ReaderT
> type Eval3 a = ReaderT Env (ErrorT String Identity) a
> he uses 'ask' and 'throwError' in some example code. I notice that
> he doesn't have to lift throwError into the ErrorT monad. Why is
> this? Do I misunderstand something about monad transformers? Is it a
> convenience definition of throwError?
Look for definitions of the form
> instance MonadReader Eval3 where ...
> instance MonadError Eval3 where ...
These effectively "import" definitions of ask, throwError, etc. with
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university KF8NH
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