The document discusses techniques for structuring Haskell programs using monads and monad transformers in a way that is clear and focuses on essential ideas. It presents an example of refactoring a program to use type classes to represent dependencies instead of passing explicit configuration everywhere. This improves readability and hides implementation details. The approach demonstrated is called "MTL-style" or "finally tagless" and provides a clean separation of interfaces and implementations.

1. MonadReader: Less is More
Sukant Hajra / @shajra / CognitiveScale
May 27, 2017
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 1 / 17

2. For Time, Not Covered
Monads
Monad Transformer Basics
Monad Transformer Library (MTL)
Optics (Lenses)
Classy Optics
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 2 / 17

3. Some Motivation
Figure 1: An Internet Joke by Matt Parsons
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 3 / 17

4. Exercise 1
Which one?
A
1parseConfig `bind` runApp
B
1parseConfig >>= runApp
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 4 / 17

5. Exercise 2
A
The Bobolinks emDash begun emDash newLine Then I said softly to myself
emDash newLine openDoubleQuotes That must have been the Sun
closeDoubleQuotes exclamationPoint
B
The Bobolinks – begun –
Then I said softly to myself –
“That must have been the Sun”!
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 5 / 17

6. Is the Naïve also Simple?
1makeUser :: Config -> IO ()
2makeUser = ...
4getInsight :: Config -> IO ()
5getInsight = ...
7runApp :: Config -> IO ()
8runApp = do
9user <- makeUser
10insight <- getInsight(user)
11showInsight(user, insight)
Complaint
passing around a global Config everywhere
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 6 / 17

7. Classy Optics
Does this help?
1makeUser
2:: HasDbConfig c
3=> c -> IO ()
5getInsight
6:: (HasDbConfig c, HasMetricsConfig c)
7=> c -> IO ()
Complaint
IO lock-in
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 7 / 17

8. Sometimes libraries force our hand
1makeUser
2:: (HasDbConfig c, MonadIO m)
3=> c -> m ()
5getInsight
6:: (HasDbConfig c, HasMetricsConfig c, MonadIO m)
7=> c -> m ()
Some Complaints
It’s getting kind of busy
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 8 / 17

9. Is MTL worth it?
1makeUser
2:: (HasDbConfig c, MonadReader c m, MonadIO m)
3=> m ()
5getInsight
6:: ( HasDbConfig c, HasMetricsConfig c
7, MonadReader c m, MonadIO m )
8=> m ()
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 9 / 17

10. Make the API reflect how you think!
Component-centric type classes
1class MonadIO m => Db m where
2getDbConn :: m DbConn
4class Monad m => Metrics m where
5sendMetric :: Component -> Event -> m ()
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 10 / 17

11. Say what you mean. . .
No more, no less
1makeUser :: Db m => m ()
3getInsight :: (Db m, Metrics m) => m ()
Observations/Comments
Parses better at a glance
Implementations decisions are hidden/deferred
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 11 / 17

12. We’re not quite done
We still need an implementation
1{-# LANGUAGE GeneralizedNewtypeDeriving #-}
3newtype App a = App { unApp :: ReaderT Config IO a }
4deriving
5( Functor , Applicative , Monad
6, MonadIO, MonadReader Config )
8instance Db App where
9getDbConn = ask >>= liftIO . libMakeDbConn . getDbUrl
11instance Metrics App where
12sendMetric c s = ask >>= liftIO . libSendMetric c s
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 12 / 17

13. This is not new
Is there a name for this?
“MTL-style”
“finally tagless”
Good example code
Katip (a Haskell logging library)
github.com/Soostone/katip/blob/master/katip/examples/example.hs
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 13 / 17

14. Thanks
Remember
Reward human beings
Don’t punish not-machines
Some techniques
Maximize comprehension at a glance.
Keep high-level ideas visible.
Reduce contrast of high-level with low-level ideas.
Whitespace matters.
Sukant Hajra / @shajra / CognitiveScale MonadReader: Less is More May 27, 2017 14 / 17

15. Quick mention
Figure 2: My Employer
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16. Extra Material Not Presented
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17. Teaser for a follow-up discussion
Problems we’ve had with MonadError
MonadError locks error type
Need to call runExceptT to handle exceptions
Inconvenient with a nested stack
Requires intermediate error types for matching
We’ve ended up with this kind of code
1runApp :: (Db m, Metrics m) => ExceptT SomeAPIFault m ()
Experimenting with a new approach
Explicitly Typed Exceptions for Haskell by Josè Iborra
Seems promising in theory and small examples
Want to use more at scale before recommending
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