[Haskell-cafe] Most Important GHC extensions to learn/use?

wren ng thornton wren at freegeek.org
Fri Jun 1 06:29:56 CEST 2012

On 5/31/12 7:15 PM, Jonathan Geddes wrote:
> Haskell Hackers,
> I'm pretty comfortable with all of Haskell 98 (and 2010, really). But I've
> always sort of avoided extensions. I realize that this is a bit silly and
> if I want to continue learning, it probably means delving into the
> extensions. Which ones are the most important to know from a practical
> point of view? And which ones from a {Language,Category,Math}-theoretical
> point of view? (Any other interesting/important points of view I'm missing?
> :D )

There are a bunch which are mostly just syntax changes. The important 
ones are:

     ForeignFunctionInterface (aka FFI)
         Not technically part of H98, though it was a quick addition. It
         is part of H2010, so it's not really an "extension" anymore.

         This one's really easy, and in the cases where you want it you
         really really want it.

         This one's simple, and it helps expose you to the idea of
         kinds, which is helpful for what's to come.

         This one's trivial, but it makes things a bit prettier.

     FlexibleContexts, FlexibleInstances
         These are essential for actually using MPTCs (described below).
         IMO they should be enabled automatically whenever MPTCs are on.

And there are also a bunch of ones about extending the "deriving" 
mechanic to work with new classes or with newtypes.

Then there are the ones that actually change the language in a 
significant way. I'd say the critical ones to learn are:

     RankNTypes (or Rank2Types if you're squeamish)
         This is used in lots of nice tricks like list fusion. Learning
         list fusion is a good place for the H98 veteran to explore
         next, since it's easy to pick up and has many applications
         outside of just doing list fusion. Also, it's been around
         forever and isn't going anywhere anytime soon.

     MultiParamTypeClasses (aka MPTCs)
         This has been around forever, and is considered "standard
         Haskell" by most people, even though it hasn't made it into the
         Report yet (due the the fundeps vs TFs issue).

     FunctionalDependencies (aka fundeps)
         This is helpful for making certain MPTCs usable without too
         many type signatures. Also, it's good for understanding the
         fundeps vs TFs issue. Also, this one has been around forever,
         and although it's fallen into disfavor it is still
         indispensable due to limitations in TFs.

     TypeFamilies (aka TFs)
         These are really nifty and they're all the rage these days. In
         a formal sense they're equivalent to fundeps, but in practice
         they're weaker than fundeps.

         These are really nifty and they're all the rage these days.
         Though beware, GADTs are a rabbit hole leading off to the world
         of dependent types. You should be aware of the basic ideas
         here, though don't worry too much about the theory (unless you
         want to spend a lot of time worrying about the theory).

Live well,

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