[Haskell-cafe] Is there an escape from MonadState+MonadIO+MonadError monad stack?
rendel at informatik.uni-marburg.de
Sun Apr 7 00:34:00 CEST 2013
Ömer Sinan Ağacan wrote:
> One thing I'm not happy about my Haskell programs is, almost all of my
> programs have a monad transformer stack consisting MonadError, MonadIO
> and MonadState.
You can try to write most of your program in pure functions that are
called from a few "main" functions in the monad. Or, if you need some
but not all monadic actions in each function, you can use the following
-- This helper function cannot cause monadic effects other than
-- throwing errors. But it can be used in arbitrary monads that
-- support throwing errors.
helper :: MonadError MyError m => ... -> m ...
helper = do ...
-- Same but with only allowing IO, but other monadic actions
other :: MonadIO m => ... -> m ...
other = do ...
-- we can use both functions in the same monad
main = runMyStack $ do
This way, you have some control over what effects are allowed where.
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