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Getting it Donewith Haskell

From Elegance to Engineering

Greg Weber - doxIQBayHac 2014

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Motivation

You already have an understanding of H

You want some suggestions and guidan

how to effectively write useful programs

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Real World Haskell

Takes time to show concrete

instance of building real-world code.

Starting to get dated. Haskell libraries and best prahave evolved a lot in certain areas since RWH waspublished.

For example, the chapters on databases now feelslevel and their is a lack of information on web servprogramming.

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Beginning Haskell

Kind of a modern RWH

● More beginner focus● Less of a deep dive into real world code

● Smaller code samples● Shows how to use modern libraries● Missing some things (example: command line, ● Looks good to me, but I am no longer a beginn

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Parallel & Concurrent ...

Great on its topic Author created async package

This topic spans most of the benefitsof the GHC implementation.

● async IO, but write in a blocking style

● STM

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Typeclassopedia

Learn (most of) this!

opinion: Don’t bother with Arrow* until la

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Typeclasses + Data Structures

composition & re-useapply abstract typeclasses to

concrete Data structures

This cheatsheet gives a great overview

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mono-traversable

Extends the concept ofsome of the coretypeclasses (Functor,Foldable, Traversable) towork on Monomorphic types(Text, ByteString, etc).

type family Element mon

type instance Element T.

instance MonoFunctor T.  omap = T.map

instance MonoFoldable T  ofoldMap f = ofoldr (ma  ofoldr = T.foldr   ofoldl' = T.foldl'  ...

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Error Handling

Pure code

  * do not throw an exception, ever!  * Maybe/Either 

  * Use ErrorT/EitherT to break out if needed

IO: you must deal with exceptions alread● if you are doing throwError "string", you really j

2 separate types of error mechanisms to handl● instead, just use exceptions from the exception

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Threads + Exceptions

By default, threads die silently

 Add a default exception handler 

Link your threads together link :: Async a -> IO ()

Link the given Async to the current thread, such that if the Async raises an exception, that exception will be re-thrown in t

myThreadId … forkIO $ catchAny … throwTo

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Avoid errors altogether 

-- Data.List

-- watch out for the hidden exception!

head :: [a] -> a

head (x: _ )  =  xhead [] =  error "head: empty list"

-- List has an information loss

x : y : []

-- Data.List.NonEmpty (s

head :: NonEmpty a -> a

head (a :| _ ) = a

-- Prove your data structu

 -- NonEmpty knows ther

x :| (y : [])

-- Data.NonNull from mono-travesable. Apply NonEmpty concept to an

-- Also, use type-level numbers to be a little smarter.

head :: MonoTraversable mono => MinLen (Succ nat) mono -> Elemen

head :: MinLen (Succ nat) (Seq a) -> a

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Prefer NonEmpty to safesafe provides exception free functions or exceptionfunctions that you can provide useful error messageswith

By giving a unique error message with the exactfunction name, we can now track down the error.

But we already said: never throw an exception in purecode! (It can actually be ok to do this in your application, just don’t

share that code with anyone).

Prelude> head []

*** Exception: Prlist

-- headNote :: St

Safe> headNote

*** Exception: Pahead [], bad func

-- headMay :: [a

Safe> headMay

Nothing

So lets use headMay. But now we have to deal with the Maybe.The problem is we decided to drop information and defer figuring thinglater.

Instead, encode information in your program when you first have it.

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classy-prelude

no partial functionssome API cleanup

  * constant memory implementations (fo

mapM_)  * better grouping functionality

good exception handling tools by default

mono-traversable

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Libraries

Why you may not be more productive in

Haskell

But there are so many libraries now!They aren’t always as polished as their counterparts in other languamore likely to be in a quasi-abandoned state.

● Be prepared to dig into any library you use andpull request.

● Be prepared to maintain some libraries

● Be prepared to write new libraries

W iti / i t i lib i

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Writing /maintaing librariesGregWeber is part of the f ollowing groups:

● Maintainer s for azurify● Maintainer s for cabal-meta● Maintainer s for css-text

● Maintainer s for dlist-instances● Maintainer s for file-location● Maintainer s for fsnotif y● Maintainer s for hamlet● Maintainer s for heroku● Maintainer s for hspec-expectations-pretty● Maintainer s for hspec● Maintainer s for monad-logger-syslog● Maintainer s for mongodb-queue● Maintainer s for par se-help

● Maintainer s for persistent-mongoDB● Maintainer s for persistent● Maintainer s for rollbar ● Maintainers for shakespeare● Maintainers for shelly-extra● Maintainers for shelly● Maintainers for spelling-suggest● Maintainers for sphinx● Maintainers for wai-static-pages● Maintainers for wai-test● Maintainers for xss-sanitize

● Maintainers for yesod-bin● Maintainers for yesod-core

In a team of Haskell programmeleast one member  that works fluesource community.

Stop creating an artificial dividingapplication and the libraries you running the code, you need to ow

Learn to vendor the libr aries that

If there is a bug in a library, it is yand probably to fix it if it is easy.

If you need a feature, it is your jotalking with the maintainers).

Doing this is perhaps the best wa

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Choosing libraries

● search google, ask questions, see

announcements● goto hackage.haskell.org● goto github page and look at README.● view the haddock documentation● look at the source code● look at the author (but don’t cargo cult)● look at the dependencies (may need to

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Testing

QuickCheck: brilliant: confirm your propetasty or hspec: organize your test suites

ran out of time for this slide

C b l

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Cabal

Jonathan already talked about that at Bay

 Application developers, do this:cabal sandbox init && cabal install && cabal freeze

Library authors should not freezeDependencies that are not on hackage:cabal sandbox add-source ../vendor/lib

cabal install --only-dep

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Development environment

Integrate these with your text editor/workstylish-haskell, hlint, ghc-mod, hasktags, codex

local hoogle (hoogle data)

online hayoo & hoogle (+module)

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Command line parsing

Use optparse-applicative* combinators, decent static assurances

* help output

* command line completion

ran out of time ...

Sh ll S i ti ith Sh ll

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Shell Scripting with Shelly

  {-# LANGUAGE OverloadedStrings #-}

  {-# LANGUAGE ExtendedDefaultRules #-}

  {-# OPTIONS_GHC -fno-warn-type-defaults #-}

  import Shelly

  import Data.Text as T

  default (T.Text)

  main = shelly $ verbosely $ do

  d <- cmd "date"

  c <- escaping False $

  cmd "git" "log -1 | head -1 | awk '{print $2}'"

  appendfile "log/deploy.log" $ T.intercalate " - " [T.stripEnd d, c]

Using shakespeare-text

From yesod developme

errorExit [lt|

  installation failure!

  Please try a clean build with: ./script/install

  If  you are peforming a clean install and have

  Please report this error to the mail list at htcom/group/yesodweb

  or on the issue tracker at http://github.com/

|]

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