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.
ScopedTypeVariables
This one's really easy, and in the cases where you want it you
really really want it.
KindSignatures
This one's simple, and it helps expose you to the idea of
kinds, which is helpful for what's to come.
TypeOperators
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.
GADTs
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,
~wren
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