# Specific topics for Haskell users

### What does that `<-` / `do` / list comprehension syntactic sugar do?

Excellent [article](http://www.haskellforall.com/2014/10/how-to-desugar-haskell-code.html).

### For understanding list and fold

- [Explain List Folds to Yourself](http://vimeo.com/64673035)

### For learning some common typeclasses

Useful for understanding `Functor`, `Applicative`, `Monad`, `Monoid` and other
typeclasses in general but also some Hask-specific category theory:

- The [Typeclassopedia](http://www.haskell.org/haskellwiki/Typeclassopedia)

### Understanding basic Haskell error messages

- [Understanding basic error messages](http://ics.p.lodz.pl/~stolarek/_media/pl:research:stolarek_understanding_basic_haskell_error_messages.pdf)

---

# Laziness, strictness, guarded recursion

- Heinrich Apfelmus' article series at https://hackhands.com/guide-lazy-evaluation-haskell/ is excellent.

- Marlow's [book](http://chimera.labs.oreilly.com/books/1230000000929/ch02.html)
  about parallelism and concurrency has one of the best introductions to
  laziness and normal form I've found. Use other material too if it doesn't
  stick immediately.

- [More points for lazy evaluation](http://augustss.blogspot.hu/2011/05/more-points-for-lazy-evaluation-in.html)

- [Oh my laziness!](http://alpmestan.com/posts/2013-10-02-oh-my-laziness.html)

- SO question '[Does haskell have laziness?](https://stackoverflow.com/questions/13042353/does-haskell-have-tail-recursive-optimization)'

- [Johan Tibell](https://github.com/tibbe)'s slides from a talk on
  [reasoning about laziness](http://www.slideshare.net/tibbe/reasoning-about-laziness).

## Brief demonstration

```haskell
let a = 1 : a -- guarded recursion, (:) is lazy and can be pattern matched.
let (v : _) = a
> v
1
> head a -- head a == v
1

let a = 1 * a -- not guarded, (*) is strict
> a
*** Exception: <<loop>>
```

# IO

- [Evaluation order and State tokens](https://www.fpcomplete.com/user/snoyberg/general-haskell/advanced/evaluation-order-and-state-tokens)

- [Unraveling the mystery of the IO monad](http://blog.ezyang.com/2011/05/unraveling-the-mystery-of-the-io-monad/).

- [First class "statements"](http://blog.jle.im/entry/first-class-statements).

- [Haddocks for System.IO.Unsafe.unsafePerformIO](http://hackage.haskell.org/package/base-4.7.0.1/docs/System-IO-Unsafe.html#v:unsafePerformIO)
  Read the docs and note implementation of unsafeDupablePerformIO

Comment from Reddit thread by `glaebhoerl`

> Interesting side note: GHC needs to hide the state token representation behind
> an abstract IO type because the state token must always be used linearly (not
> duplicated or dropped), but the type system can't enforce this. Clean, another
> lazy Haskell-like language, has uniqueness types (which are like linear types
> and possibly different in ways I'm not aware of), and they expose the
> World-passing directly and provide a (non-abstract) IO monad only for
> convenience.

# Monads and monad transformers

> Do not do these until you understand typeclasses, Monoid, Functor, and
> Applicative!

Implement the standard library monads ( List, Maybe, Cont, Error, Reader,
Writer, State ) for yourself to understand them better. Then maybe write an
monadic interpreter for a small expression language using
[Monad Transformers Step by Step](https://page.mi.fu-berlin.de/scravy/realworldhaskell/materialien/monad-transformers-step-by-step.pdf)
paper (mentioned in 'monad transformers' below).

Writing many interpreters by just changing the monad to change the semantics can
help convey what's going on.

Also, reimplement `Control.Monad`. Functions like `mapM` or `sequence` are good
opportunities to practice writing generic monadic code.

The FP course can be used as a guide to this process, which will also involve
writing your own Applicative as well.

Credits:

- Reddit comment by htmltyp and Crandom [here](https://www.reddit.com/r/haskell/comments/29eke6/basic_program_ideas_for_learning_about_monads/cik5aj6).

- Reddit comment by jozefg [here](https://www.reddit.com/r/haskell/comments/29eke6/basic_program_ideas_for_learning_about_monads/cik5trg).

## Monad transformers

- [A gentle introduction to Monad Transformers](https://github.com/kqr/gists/blob/master/articles/gentle-introduction-monad-transformers.md).

- [Monad transformers step-by-step](https://page.mi.fu-berlin.de/scravy/realworldhaskell/materialien/monad-transformers-step-by-step.pdf).

# Testing, tests, specs, generative/property testing

- This [tutorial](https://github.com/kazu-yamamoto/unit-test-example/blob/master/markdown/en/tutorial.md) by Kazu Yamamoto is fantastic.

- [Simple-Conduit](https://github.com/jwiegley/simple-conduit): Good simple
  library for learning how streaming IO works in general, knowledge
  transferrable to libraries like Pipes and Conduit

# Parsing in Haskell

- Parser combinator [tutorial](https://github.com/JakeWheat/intro_to_parsing)
  for Haskell using Parsec

- [Writing your own micro-Parsec](http://olenhad.me/articles/monadic-parsers/)

## Parsing and generating JSON

Aeson is the standard [JSON](https://json.org) parsing solution in
haskell. Available from [hackage](https://hackage.haskell.org/package/aeson) and
[github](https://github.com/bos/aeson).

- [Parsing JSON using Aeson](http://blog.raynes.me/blog/2012/11/27/easy-json-parsing-in-haskell-with-aeson/)

- [Aeson and user created types](http://bitemyapp.com/posts/2014-04-11-aeson-and-user-created-types.html)

- [Parsing non-deterministic data with aeson and sum types](http://bitemyapp.com/posts/2014-04-17-parsing-nondeterministic-data-with-aeson-and-sum-types.html)

- [Aeson tutorial](https://www.fpcomplete.com/school/starting-with-haskell/libraries-and-frameworks/text-manipulation/json)

# Graph algorithms and data structures

- The [fgl package](https://hackage.haskell.org/package/fgl) particularly the
  purely functional shortest path [algos](http://hackage.haskell.org/package/fgl-5.4.2.2/docs/Data-Graph-Inductive-Query-SP.html).

- [Inductive graphs and Functional Graph Algorithms](http://web.engr.oregonstate.edu/~erwig/papers/abstracts.html#JFP01).

- [FGL/Haskell - A Functional Graph Library](http://web.engr.oregonstate.edu/~erwig/fgl/haskell/old/fgl0103.pdf).

- [Data.Graph source from Containers package](http://hackage.haskell.org/package/containers-0.5.5.1/docs/Data-Graph.html).

- The [graphs package](https://hackage.haskell.org/package/graphs).

- [SO question about PHOAS](https://stackoverflow.com/questions/24369954/separate-positive-and-negative-occurrences-of-phoas-variables-in-presence-of-rec)

- [PHOAS for free](https://www.fpcomplete.com/user/edwardk/phoas).

- [Tying the Knot](http://www.haskell.org/haskellwiki/Tying_the_Knot).

- [Hackage: dag](https://hackage.haskell.org/package/dag).

# Development Environment

## Emacs

- [Alejandro Serras's tutorial](https://github.com/serras/emacs-haskell-tutorial/blob/master/tutorial.md)

- [My dotfiles](https://github.com/bitemyapp/dotfiles/)

- [Chris Done's emacs config](https://github.com/chrisdone/chrisdone-emacs)

## Vim

- [Vim page on haskellwiki](http://www.haskell.org/haskellwiki/Vim)

- [Haskell-vim-now](https://github.com/begriffs/haskell-vim-now)

- [GHC-Mod](https://github.com/kazu-yamamoto/ghc-mod)

- [GHC-Mod vim plugin](https://github.com/eagletmt/ghcmod-vim)

- [Hindent](https://github.com/chrisdone/hindent)

## Sublime Text

- [SublimeHaskell](https://github.com/SublimeHaskell/SublimeHaskell)

# Working with Cabal

## Cabal guidelines

Cabal Hell was a problem for Haskell users before the introduction of
sandboxes. Installing outside of a sandbox will install into your user
package-db. This is *not* a good idea except for foundational packages like
Cabal, alex, and happy. Nothing else should be installed in the user or global
package-dbs unless you know what you're doing.

Some best practices for avoiding cabal hell are available
[here](http://softwaresimply.blogspot.com/2014/07/haskell-best-practices-for-avoiding.html).

To experiment with a package or start a project, begin by doing
`cabal sandbox init` in a new directory.

Put briefly:

- Always use sandboxes for installing new packages, building new or existing
  projects, or starting experiments

- Use `cabal repl` to start a project-scoped ghci instance

The sandbox-based approach I suggest should avoid package-dependency problems,
but it's incompatible with the way the Haskell Platform provides pre-built
packages. If you're still learning Haskell and don't understand how ghc-pkg and
Cabal work, *avoid platform* and instead use the install instructions earlier in
the guide.

## Stackage

For any users (usually Yesod users) that have build problems, consider Stackage.

- A good summary of Stackage is
  [here](https://www.fpcomplete.com/blog/2014/05/stackage-server).

In the author's opinion, Stackage is usually more useful than `cabal freeze`.

# Hoogle and Haddock

## Search code by type signature

The [Hoogle search engine](http://www.haskell.org/hoogle/) can search by type.

For example, look at the search results for `(a -> b) -> [a] -> [b]`
[here](http://www.haskell.org/hoogle/?hoogle=%28a+-%3E+b%29+-%3E+%5ba%5d+-%3E+%5bb%5d).

Also hosted by fpcomplete [here](https://www.fpcomplete.com/hoogle).

Also [Hayoo](http://holumbus.fh-wedel.de/hayoo/hayoo.html) (which has all of
hackage enabled for search by default).

## Setting up your own local instance of Hoogle

Take a look [here](https://gist.github.com/bitemyapp/3e6a015760775e0679bf).

## Haddock

1. [Fix your hackage documentation](http://fuuzetsu.co.uk/blog/posts/2014-01-06-Fix-your-Hackage-documentation.html)

2. [Hackage documentation v2](http://fuuzetsu.co.uk/blog/posts/2014-01-06-Hackage-documentation-v2.html)

Note that these posts are *slightly out of date*: for example, now Hackage sports
shiny new info with documentation info and build status.

## What you really need to know

In order to have haddocks include documentation for related packages, you have
to set `documentation: True` in your `~/.cabal/config`. If it was left on the
default (`False`) or set to `False`, you'll have to delete all your packages and
reinstall before generating haddocks.

The other thing to keep in mind is that due to the way the `$pkg` parameter gets
interpolated *by* cabal, not by you, the `html-location` and `content-location`
parameters *must be in single quotes* and entered into a shell or contained in a
shell script. They will not work in a Makefile, because it will think they are
Make variables!

```bash
#! /usr/bin/env sh

# You can write it one one line by skipping the backslashes
cabal haddock --hoogle --hyperlink-source                       \
 --html-location='http://hackage.haskell.org/package/$pkg/docs' \
 --contents-location='http://hackage.haskell.org/package/$pkg'
```

# TravisCI

If you're as big a fan of [TravisCI](https://travis-ci.org) as I am, then I
*strongly* recommend you take a look at
[multi-ghc-travis](https://github.com/hvr/multi-ghc-travis) by as the basis of
the `travis.yml` for your Haskell projects.

# Frontend/JavaScript

We have an embarrassment of riches! There are three main choices I would
recommend:

* [Haste](http://haste-lang.org/) a Haskell to JavaScript compiler
  - The [compiler](https://github.com/valderman/haste-compiler) on github.
  - An excellent
    [demo](http://www.airpair.com/haskell/posts/haskell-tutorial-introduction-to-web-apps)
    of Haste with an example project.

* [GHCJS](https://github.com/ghcjs/ghcjs)
  - [GHCJS Introduction](http://weblog.luite.com/wordpress/?p=14)
  - [Functional Reactive Web Interfaces with GHCJS and Sodium](http://weblog.luite.com/wordpress/?p=127)
  - [Writing Atom plugins in Haskell using ghcjs ](http://edsko.net/2015/02/14/atom-haskell/)

* [PureScript](http://www.purescript.org/)
  - Not strictly Haskell like Haste and GHCJS, but a popular choice among
    Haskellers
  - Written in and inspired by haskell
  - Try purescript in you browser [here](http://try.purescript.org/)
  - Great guide for [getting started](http://www.christopherbiscardi.com/2014/06/22/getting-started-with-purescript/)

## Which frontend language do I use?

GHCJS and Haste are both fully Haskell. GHCJS will work with more Haskell
packages than Haste, but this doesn't affect a lot of frontend
projects. PureScript isn't Haskell at all, so direct code sharing with your
backend will not work.

GHCJS has the fattest runtime payload overhead at about 100kb (luite is working
on this). Haste and PureScript are competitive.

PureScript has the best JS tooling integration (uses gulp/grunt/bower), GHCJS
and Haste integrate better with Haskell's tooling (Cabal).

All three are great choices and will work for most frontend projects.

# For a more thorough understanding of laziness, NF, WHNF

- [Notes on lambda calculus](https://vec.io/posts/notes-on-lambda-calculus).

## Research papers about lazy lambda calculi

- [A call by need lambda calculus](http://homepages.inf.ed.ac.uk/wadler/topics/call-by-need.html#need-journal).

- [Demonstrating Lambda Calculus Reduction](http://www.itu.dk/~sestoft/papers/sestoft-lamreduce.pdf)

- [The lazy lambda calculus](http://www.cs.ox.ac.uk/files/293/lazy.pdf).

- [Lazy evaluation of Haskell](http://www.vex.net/~trebla/haskell/lazy.xhtml)

# Parallelism/Concurrency

- [Parallel and Concurrent Programming in Haskell](http://chimera.labs.oreilly.com/books/1230000000929). This
  book by Simon Marlow is probably the best I've ever read on the topics of
  Parallelism and Concurrency.

- A thorough [walk-through](http://kukuruku.co/hub/haskell/haskell-testing-a-multithread-application)
  on testing & incremental development of a multi-threaded application in
  Haskell.

- [Functional Reactive Programming](http://www.haskell.org/haskellwiki/Functional_Reactive_Programming)

# Lenses and Prisms

After you're comfortable with Haskell, strongly consider learning Lenses and
Prisms, even if just as a "user". You don't need to understand the underlying
category for it to be useful.

People vastly overestimate the difficulty of using Lens. Anybody comfortable
with Functor/Foldable/Traversable (or even just the first one) can leverage
lenses and prisms to make their life happier.

If you've ever done something like: `(fmap . fmap)` you were "lensing" in your
head.

I recommend these two tutorials/introductions:

- [A little lens starter tutorial](https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/a-little-lens-starter-tutorial)

- [Lens: Lenses, Folds and Traversals](https://github.com/ekmett/lens#lens-lenses-folds-and-traversals)

Look here for more information: [Lens package on hackage](http://hackage.haskell.org/package/lens).

# Recursion Schemes

Some of the crazy \*-morphism words you've heard are actually about
recursion. NB - before tackling this material you should know how to implement
foldr for lists and at least one other data structure, such as a tree. (folds
are catamorphisms) Knowing how to implement an unfold (anamorphism) for the same
will round things out a bit.

This material dovetails with traversable and foldable.

- [An introduction to recursion schemes](http://patrickthomson.ghost.io/an-introduction-to-recursion-schemes/)

- [Don't fear the cat](http://fho.f12n.de/posts/2014-05-07-dont-fear-the-cat.html) -
  Good demonstration of how hylomorphism is the composition of cata and ana.

- [Recursion Schemes](http://comonad.com/reader/2009/recursion-schemes/) - This
  field guide is excellent.

- [Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire](http://eprints.eemcs.utwente.nl/7281/01/db-utwente-40501F46.pdf)

- [Catamorphisms](https://www.fpcomplete.com/user/edwardk/recursion-schemes/catamorphisms)

# GHC Core and performance tuning

- [Write Haskell as Fast as C](write_haskell_as_fast_as_c.md)

- [GHC Wiki: CoreSyn Type](https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/CoreSynType).

- [Hackage: GHC Core](https://hackage.haskell.org/package/ghc-core).

- [SO Question: Reading GHC Core](https://stackoverflow.com/questions/6121146/reading-ghc-core).

- [Haskell as fast as C](http://donsbot.wordpress.com/2008/06/04/haskell-as-fast-as-c-working-at-a-high-altitude-for-low-level-performance/).

- [Real World Haskell, Chapter 25: Profiling and Optimizations](http://book.realworldhaskell.org/read/profiling-and-optimization.html).

# Type and Category Theory

> *Not* needed to actually write Haskell, just for those interested!

If you want to follow up on type and category theory:

- [Catster's Guide](http://byorgey.wordpress.com/2014/01/14/catsters-guide/) and
  [Catster's Guide 2](http://byorgey.wordpress.com/catsters-guide-2/)

- The [haskell wikibook](http://en.wikibooks.org/wiki/Haskell/Category_theory)
  has nice diagrams

- [Category Theory](http://www.haskell.org/haskellwiki/Category_theory) on
  haskellwiki, also has good links to other resources

- [Categories from scratch](http://science.raphael.poss.name/categories-from-scratch.html), Includes some practical examples.

- Pierce's [Great Works in PL](http://www.cis.upenn.edu/~bcpierce/courses/670Fall04/GreatWorksInPL.shtml) list.

## Books

- [Quora Question: What is the best textbook for category theory?](http://www.quora.com/Category-Theory/What-is-the-best-textbook-for-Category-theory?share=1) Kmett's recommendations

- [Awodey](http://ukcatalogue.oup.com/product/9780199237180.do) and
  [MacLane](http://www.amazon.com/Categories-Working-Mathematician-Graduate-Mathematics/dp/0387984038). The standard textbooks on category theory.

- [Harper's Practical Foundations for Programming Languages](http://www.cs.cmu.edu/~rwh/plbook/1sted-revised.pdf) is the best PL focused intro to type theory I've read.

- [Type theory and Functional Programming](http://www.cs.kent.ac.uk/people/staff/sjt/TTFP/).

# Metaprogramming

## Generics

Generics are usually used to generate typeclass instances, but you'll see TH used for that purpose too.

- https://ocharles.org.uk/blog/posts/2014-04-26-constructing-generically.html

- Current docs: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#generic-programming

## QuasiQuotation

- Simple example, nice for avoiding gnarly escape sequences in strings: https://hackage.haskell.org/package/raw-strings-qq-1.0.2/docs/Text-RawString-QQ.html

- Tutorial: https://www.fpcomplete.com/user/marcin/quasiquotation-101

- http://www.well-typed.com/blog/2014/10/quasi-quoting-dsls/

## Template Haskell

- http://www.parsonsmatt.org/programming/2015/11/15/template_haskell.html (Great tutorial, strongly recommended even if the Ruby doesn't mean much to you)

- https://www.fpcomplete.com/user/marcin/template-haskell-101

- https://ocharles.org.uk/blog/guest-posts/2014-12-22-template-haskell.html

- Current docs: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#template-haskell

- OCaml and TH slides: https://nicolaspouillard.fr/talks/cufp-metaprogramming-tutorial-slides.pdf

- http://illustratedhaskell.org/index.php/2011/09/24/template-haskell-tutorial/ (old)

- https://stackoverflow.com/questions/10857030/whats-so-bad-about-template-haskell (warnings about TH)


# Other fun topics

## Parametricity, ad-hoc vs. parametric polymorphism, free theorems

- [Parametricity](tony_parametricity.pdf).

- [TeX sources](https://github.com/tonymorris/parametricity/) for the
  above talk.

- [Making ad-hoc polymorphism less ad-hoc](http://swizec.com/blog/week-20-making-ad-hoc-polymorphism-less-ad-hoc/swizec/6564).

- [Theorems for Free!](http://ttic.uchicago.edu/~dreyer/course/papers/wadler.pdf).

## Initial and Final, DSLs, Finally Tagless

- [Final Encodings, Part 1: A Quick Demonstration](http://creativelad.wordpress.com/2013/11/28/final-encodings-part-1-a-quick-demonstration/).

- [Transforming Polymorphic Values](http://martijn.van.steenbergen.nl/journal/2009/10/18/transforming-polymorphic-values/).

- [GADTs in Haskell 98](http://martijn.van.steenbergen.nl/journal/2009/11/12/gadts-in-haskell-98/).

- [Typed Tagless-Final Linear Lambda Calculus](https://www.fpcomplete.com/user/mutjida/typed-tagless-final-linear-lambda-calculus).

- [Typed tagless-final interpretations: Lecture notes](http://okmij.org/ftp/tagless-final/course/course.html).

- [Typed Tagless Final Interpreters](http://okmij.org/ftp/tagless-final/course/lecture.pdf).

- [The dog that didn't bark](http://existentialtype.wordpress.com/2011/03/21/the-dog-that-didnt-bark/) less specifically relevant but interesting.

## Comonads

- [Comonads in Haskell](https://speakerdeck.com/dmoverton/comonads-in-haskell).

- [SO question: Can a Monad be a Comonad](https://stackoverflow.com/questions/16551734/can-a-monad-be-a-comonad).

## Yoneda / CoYoneda

- [SO question: Step-by-step explanation of coyoneda](https://stackoverflow.com/questions/24000465/step-by-step-deep-explain-the-power-of-coyoneda-preferably-in-scala-throu).

- Free monads for Less, a sequence of three articles by Edward Kmett
  * [Part 1: Codensity](http://comonad.com/reader/2011/free-monads-for-less/).
  * [Part 2: Yoneda](http://comonad.com/reader/2011/free-monads-for-less-2/).
  * [Part 3: Yielding IO](http://comonad.com/reader/2011/free-monads-for-less-3/).

## Propositions vs. Judgments (computation)

- [StackExchange question: What is the difference between propositions and judgements](http://cstheory.stackexchange.com/questions/9826/what-is-the-difference-between-propositions-and-judgments).

- [Lecture notes from a short, three lecture course](http://www.ae-info.org/attach/User/Martin-L%C3%B6f_Per/OtherInformation/article.pdf)

# Dependent typing

- [Grokking sum types, value constructors, and type constructors](http://bitemyapp.com/posts/2014-04-05-grokking-sums-and-constructors.html) squint hard.

- [Lightweight Dependent-type Programming](http://okmij.org/ftp/Computation/lightweight-dependent-typing.html).

- [Idris programming language](http://www.idris-lang.org/).

# Statically linking binaries

- [Static linking](https://wiki.haskell.org/Web/Literature/Static_linking)

- [Static linking with GHC on Arch Linux](http://www.edofic.com/posts/2014-05-03-ghc-arch-static.html)

- [Statically linking Linux binaries for ARM & MIPS](https://stackoverflow.com/questions/14270177/ghc-statically-linking-linux-binaries-for-arm-mips-processors)

- [Statically link GMP using GHC and LLVM](https://stackoverflow.com/questions/10539857/statically-link-gmp-to-an-haskell-application-using-ghc-llvm)