Haskell

Last updated July 14, 2022

• Seedling

GHCi tip: import Module and :load Module do very different things!

import brings the exports of Module into scope.

:load puts you in the context of that module, including language extensions, everything imported in that module, and non-exported terms from the module

http://lambdafoo.com/blog/2018/06/22/transformers-either/

When you see a constructor on the left hand side that’s pattern matching

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k :: Z -> String
k (X s) = s

Alternatively you can use case of

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l :: Z -> String
l Z = case Z of
  X s -> s
  Y -> "y"

similar to klass in Java. Used to escape variable bindings

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data Bool` = True` | False`

Polmrophic types in haskell always start with a lower case letter. A lower case type means polymorphic [generic]

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i :: a -> a
i x = x

Gentle reminder to any #haskell learners out there still trying to wrap their heads around Functor/Applicative/Monads:

1. Just write code
2. Read https://t.co/3ISK3zLjoc
3. Repeat until it clicks

Pretty sure I wasted a year by not doing exactly this. It really is that simple.

— Brian Jones (@mojobojo) May 16, 2018

https://qnikst.github.io/posts/2018-10-29-metrics-haskell.html

https://medium.com/e-bot7-tech/improving-performance-of-your-haskell-code-with-profiteur-8d2e025b6779 http://neilmitchell.blogspot.com/2018/10/announcing-profiterole-ghc-profile.html?m=1

• https://hackage.haskell.org/package/transformers-either
• https://github.com/expression-oriented/failing-gracefully
• https://hackage.haskell.org/package/hoist-error
• https://github.com/alephcloud/hs-hoist-error

• https://abailly.github.io/posts/haskell-reliability.html
• http://www.parsonsmatt.org/2018/03/22/three_layer_haskell_cake.html

• https://github.com/Soostone/katip/blob/master/katip/README.md

• https://github.com/abailly/hevents
• https://abailly.github.io/posts/event-source.html

• https://github.com/qfpl/validation

• https://propellor.branchable.com/

• https://abailly.github.io/posts/cm-infra-1.html
• https://abailly.github.io/posts/cm-infra-2.html
• https://propellor.branchable.com/

• https://abailly.github.io/posts/cm-arch-design.html

• http://blog.akii.de/posts/2018-02-19-geoip-lookup-part-1.html
• https://abailly.github.io/posts/note-on-stm.html

• https://simonmar.github.io/pages/pcph.html

• http://www.scs.stanford.edu/11au-cs240h/notes/ghc.html

• https://github.com/bfpg/cis194-yorgey-lectures

• https://github.com/dmjio/miso/blob/master/README.md

• https://abailly.github.io/posts/cm-arch-design.html
• http://blog.akii.de/posts/2017-06-04-eventsourcing-in-haskell.html
• https://abailly.github.io/posts/dependent-types-ddd.html

• http://hackage.haskell.org/package/http-client-restricted-0.0.1/docs/Network-HTTP-Client-Restricted.html

• https://abailly.github.io/posts/existential-types.html

applications.md

• https://github.com/ahushh/Monaba

• https://github.com/yamadapc/yesod-servant-example

• https://haskell-servant.github.io/posts/2016-02-06-servant-swagger.html

• http://www.parsonsmatt.org/2016/12/18/servant_in_yesod_-_yo_dawg.html c9.md

  ,—–.,–. ,–. ,—. ,–.,——. ,——. ' .–./| | ,—. ,–.,–. ,-| || o \ | || .-. \ | .—' | | | || .-. || || |' .-. |..' | | || | \ :| –, ' ‘–'| |’ ‘-’ '' '' ‘\ -' | .' / | || '--' /| —. -----'–’ ---' —-' ---' –' --'——-' `——'

Setting up haskell-vim-now on a Cloud9 workspace:

1. Start with a Blank workspace.
2. Increase resources (use at least 7GB for disk and 2GB for ram, but more is better).
3. Edit styles.css to use a powerline font:

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@import url(https://cdn.rawgit.com/wernight/powerline-web-fonts/f3821a36beeba53e6e937319d4ee636ef30a352c/PowerlineFonts.css);

4. Update apt:

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sudo apt-get update

5. Install the Haskell stack:

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curl -sSL https://get.haskellstack.org/ | sh

6. Install haskell-vim-now:

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curl -L https://git.io/haskell-vim-now > /tmp/haskell-vim-now.sh
bash /tmp/haskell-vim-now.sh

7. Enjoy!

Switch to a newer version of node with nvm:

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nvm install 6.9.2
nvm alias default 6.9.2

Uninstall any older versions:

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nvm ls
nvm uninstall 4.6.1

data61.md

Execute nix-shell to bootstrap the environment

Create hello.hs

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x :: Integer
x = 3

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ghci -ignore-dot-ghci 

:load hello.hs #loads from filesystem
:r # reload loaded modules (inc hello.hs)
:t x # obtain type signature of x

cons :: https://en.wikipedia.org/wiki/Cons

cons - head and tail

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>> :t (:.)
(:.) :: t -> List t -> List t

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>> 1 :. 2 :. 3 :. 4 :. Nil
[1,2,3,4]

The word “cons” and expressions like “to cons onto” are also part of a more general functional programming jargon. Sometimes operators that have a similar purpose, especially in the context of list processing, are pronounced “cons”. (A good example is the :: operator in ML, Scala, F# and Elm or the : operator in Haskell, which adds an element to the beginning of a list.)

generics :: type variables cons :: https://en.wikipedia.org/wiki/Cons

cons - head and tail

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>> :t (:.)
(:.) :: t -> List t -> List t

Can enable warnings when not all patterns are matched set fwarn-incomplete-patterns see https://stackoverflow.com/questions/31866379/haskell-non-exhaustive-pattern-matching-in-haskell

const

https://en.wikipedia.org/wiki/Unary_function https://hackage.haskell.org/package/base-4.10.1.0/docs/Prelude.html#v:const

• https://wiki.haskell.org/Pointfree
• http://pointfree.io/

sum' xs = foldr (+) 0 xs

Look at the signature and write down what you have (tm) and write down what you need to get to (tm)

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data Optional a =
  Full a
  | Empty
  deriving (Eq, Show)

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What I have                                                                                    
----
x :: a
f :: a -> Optional b
----
What do I need to get to?
? :: Optional b

• David
• Tim

• src/Optional.hs
• src/List.hs

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>> :i Full
data Optional a = Full a | ...
        -- Defined at src/Course/Optional.hs:15:3

education.md

• https://channel9.msdn.com/Series/C9-Lectures-Erik-Meijer-Functional-Programming-Fundamentals/Lecture-Series-Erik-Meijer-Functional-Programming-Fundamentals-Chapter-1 index.md

https://tech-blog.capital-match.com/posts/2-cpu-credits.html

https://github.com/ezyang/compact/blob/master/README.md

https://www.fpcomplete.com/blog/2017/06/readert-design-pattern

https://hackage.haskell.org/package/auto-update-0.1.4/docs/Control-AutoUpdate.html

https://www.yesodweb.com/blog/2014/08/announcing-auto-update

https://rybczak.net/2016/03/26/how-to-reduce-compilation-times-of-haskell-projects/

packages.md

https://github.com/SumAll/haskell-cached-io README.md http://blog.wuzzeb.org/full-stack-web-haskell/ tooling.md

• Comparision of IDE’s - https://github.com/rainbyte/haskell-ide-chart

https://begriffs.com/posts/2015-07-27-haskell-source-navigation.html vim.md. https://github.com/begriffs/haskell-vim-now

https://gist.github.com/gatlin/7754289

• http://www.scs.stanford.edu/11au-cs240h/notes/ghc.html