learnhaskell/dialogues.md

# Dialogues from the IRC channel or other places

## On $ and . operator

```haskell
doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther list = reverse .doubleEveryOtherForward . reverse $ list
```

```
03:28 < bitemyapp> fbernier: reverse the list, double every other number, re-reverse the list.
03:28 < bitemyapp> fbernier: the "dot" operator is just function composition.
03:28 < bitemyapp> it's nothing special, just another function.
03:28 < bitemyapp> :t (.)
03:28 < lambdabot> (b -> c) -> (a -> b) -> a -> c
03:30 < bitemyapp> fbernier: the use of $ in that function is a little idiosyncratic and unnecessary, but not problematic.
03:37 < ReinH> fbernier: there's a missing space after the . is all
03:38 < ReinH> fbernier: f x = foo $ x ==> f = foo
03:39 < ReinH> so f x = foo . bar $ x ==> f = foo . bar
03:39 < bitemyapp> fbernier: I think it's just making it point-free in this case.
03:39 < bitemyapp> @pl f x = c . b . a $ x
03:39 < lambdabot> f = c . b . a
03:39 < bitemyapp> yeah, that ^^
03:39 < bitemyapp> fbernier: identical ^^
03:40 < ReinH> fbernier: generally, when you see a $ you can wrap the things on either side with parens and get the same expression:
03:40 < ReinH> f x = foo . bar . bazz $ x ==> f x = (foo . bar . bazz) x
03:40 < ReinH> since (x) = x, ofc
03:41 < bitemyapp> @src ($)
03:41 < lambdabot> f $ x = f x
03:41 < bitemyapp> fbernier: That's the definition of $, only other thing missing is the high precedence set for it.
03:41 < ReinH> the exception is chains of $, like foo $ bar $ baz, where you have to parenthesize in the right direction
03:41 < ReinH> or the left direction, depending on how you look at it
03:42 < bitemyapp> fbernier: http://hackage.haskell.org/package/base-4.7.0.1/docs/Prelude.html ctrl-f for $ to see more
03:42 < bitemyapp> fbernier: infixr 0 is the precedence, highest there is AFAIK
03:42 < bitemyapp> fbernier: the "infixr" means it's right associative
03:42 < bitemyapp> fbernier: as opposed to infixl which would mean left associative
03:43 < ReinH> bitemyapp: or lowest, depending on how you look at it. ;)
03:43 < bitemyapp> foo $ bar $ baz ~ foo (bar (baz))
03:43 < bitemyapp> but if it was infixl
03:43 < bitemyapp> (((foo) bar) baz)
```

## Infix operators as prefix

```
04:12 < ReinH> all infix operators can be written prefix
04:12 < ReinH> with this one weird trick. Other haskellers hate him.
04:13 < bitemyapp> > ($) id 1
04:13 < lambdabot>  1
04:13 < bitemyapp> > id $ 1
04:13 < lambdabot>  1
04:13 < bitemyapp> > id 1
04:13 < lambdabot>  1
```

## Reduction, strict evaluation, ASTs, fold, reduce

```
05:00 < ReinH> pyro-: well, "reduce" already has a typeclass, depending on what you mean
05:00 < ReinH> so does "evaluation", depending on what you mean
05:02 < pyro-> ReinH: reduce is lambda calculus under strict evaluation
05:02 < ReinH> Yep, and it's also the other thing too.
05:02 < ReinH> ;)
05:03 < pyro-> :|
05:03 < pyro-> oh, like on lists?
05:04 < mm_freak_> dealing with ASTs is a real joy in haskell, because most of the code writes itself =)
```

## Continuation passing style, CPS transform

```
05:10 < pyro-> now i am writing a cpsTransform function :D
05:10 < pyro-> it already works, but the current version introduces superflous continuations
05:10 < pyro-> so i am trying to fix :D
05:10 < ReinH> pyro-: Here's a CPS transform function: flip ($)
05:11 < pyro-> i will find out about flip
05:11 < ReinH> @src flip
05:11 < lambdabot> flip f x y = f y x
05:11 < ReinH> pyro-: the essence of CPS can be described as follows:
05:11 < ReinH> :t flip ($)
05:11 < lambdabot> b -> (b -> c) -> c
05:12 < ReinH> is the type of a function which takes a value and produces a suspended computation that takes a continuation and runs it against the value
05:12 < ReinH> for example:
05:12 < ReinH> > let c = flip ($) 3 in c show
05:12 < lambdabot>  "3"
05:12 < ReinH> > let c = flip ($) 3 in c succ
05:12 < lambdabot>  4
05:13 < mm_freak_> direct style:  f x = 3*x + 1
05:13 < mm_freak_> CPS:  f x k = k (3*x + 1)
05:13 < mm_freak_> the rules are:  take a continuation argument and be fully polymorphic on the result type
05:13 < mm_freak_> f :: Integer -> (Integer -> r) -> r
05:14 < mm_freak_> as long as your result type is fully polymorphic and doesn't unify with anything else in the type signature you can't do anything wrong other than to descend
                   into an infinite recursion =)
05:14 < mm_freak_> good:  (Integer -> r) -> r
05:15 < mm_freak_> bad:  (Integer -> String) -> String
05:15 < mm_freak_> bad: (Num r) => (Integer -> r) -> r
05:15 < mm_freak_> bad:  r -> (Integer -> r) -> r
05:15 < pyro-> but flip ($) is not what i had in mind :D
05:16 < mm_freak_> that's just one CPS transform…  there are many others =)
05:16 < ReinH> No, it's probably not.
05:16 < ReinH> But other things are pretty much generalizations of that
```

```haskell
type Variable = String

data Expression = Reference Variable
                | Lambda Variable Expression
                | Combination Expression Expression

type Kvariable = String

data Uatom = Procedure Variable Kvariable Call
           | Ureference Variable

data Katom = Continuation Variable Call
           | Kreference Variable
           | Absorb

data Call = Application Uatom Uatom Katom
          | Invocation Katom Uatom

cpsTransform :: Expression -> Katom -> Call
cpsTransform (Reference r) k = Invocation k $ Ureference r
cpsTransform (Lambda p b) k = Invocation k $ Procedure p
                                             "k" $
                                cpsTransform b $ Kreference "k"
cpsTransform (Combination a b) k = cpsTransform  a $ Continuation "v" $ cpsTransform b k
```

### Later...

```
05:38 < ReinH> So for example, if you have an incredibly simple expression language like data Expr a = Val a | Neg a | Add a a
05:38 < ReinH> a (more) initial encoding of an expression would be Add (Val 1) (Neg (Val 1))
05:38 < ReinH> A (more) final encoding might be (1 - 1) or even 0
05:39 < ReinH> The initial encoding generally is more flexible (you can still write a double-negation elimination rule, for instance
05:39 < ReinH> the final encoding is less flexible, but also does more work up-front
05:40 < ReinH> More initial encodings tend to force you to use quantification and type-level tricks, CPS and pre-applied functions tend to appear more in final encodings
05:40 < ReinH> An even smaller example:
05:40 < ReinH> \f z -> foldr f z [1,2,3] is a final encoding of the list [1,2,3]
05:41 < ReinH> pyro-: I'm not really a lisper, but I'm always looking for good reading material
05:41 < ReinH> for bonus points, the foldr encoding is *invertible* as well :)
05:44 < ReinH> pyro-: the relevance is that you seem to be using the cps transform in a more initial encoding than I usually see it
05:44 < ReinH> not that this is at all bad
05:46 < bitemyapp> ReinH: where does the invertibility in the final encoding come from?
05:46 < ReinH> foldr (:) [] :)
05:46 < ReinH> it's not generally so
05:46 < bitemyapp> > foldr (:) [] [1, 2, 3]
05:46 < lambdabot>  [1,2,3]
05:47 < bitemyapp> I may not understand the proper meaning of invertibility in this case.
05:47 < bitemyapp> Do you mean invertibility from final to initial encoding?
05:47 < ReinH> Just that, yes
05:47 < bitemyapp> how would it get you back to final from initial?
05:47 < ReinH> I'm not sure if that's the correct term
05:47 < bitemyapp> I don't think it is, but the intent is understood and appreciated.
05:48 < bitemyapp> invertibility implies isomorphism, implies ability to go final -> initial -> final
05:48 < ReinH> well, there is an isomorphism
05:48 < bitemyapp> well, we've established final -> initial, where's initial -> final for this example?
05:49 < bitemyapp> I figured it was a morphism of some sort, but with only a final -> initial and not a way to get back, I wasn't sure which.
05:49 < ReinH> toInitial k = k (:) []; toFinal xs = \f z -> foldr f z xs
05:49 < bitemyapp> thank you :)
```

### Something about adjunctions. I don't know.

```
05:51 < ReinH> bitemyapp: usually one loses information going from initial to final though
05:51 < ReinH> there's probably an adjunction here
05:51 < ReinH> there's always an adjunction
05:52 < ReinH> lol of course there's an adjunction
```

## Data structures with efficient head and tail manipulation

Asker:

I am teaching myself haskell. The first impression is very good.
But phrase "haskell is polynomially reducible" is making me sad :(.
Anyway I am trying to backport my algorithm written in C. The key to
performance is to have ability to remove element from the end of a
list in O(1).
But the original haskell functions last and init are O(n).
My questions are:
1) Is last function is something like "black box" written in C++ which
perform O(1)?
So I shouldn't even try to imagine some haskell O(1) equivalent.
2) Or will optimizer (llvm?) reduce init&last complexity to 1?
3) Some people suggest to use sequences package, but still how do they
implement O(1) init&last sequences equivalent in haskell?

* * * * *

Tom Ellis:

I'm rather confused about your question.  If you want a Haskell data
structure that supports O(1) head, tail, init and last why not indeed use
Data.Sequence as has been suggested?  As for how it's implemented, it uses
the (very cool) fingertree datastructure.  See here for more details:

* * * * *

Asker:

Tom said that finger tree gives us O(1) on removing last element, but
in haskell all data is persistent.
So function should return list as is minus last element. How it could
be O(1)? This is just blows my mind...

My hypothesis is that somehow compiler reduces creating of a new list
to just adding or removing one element. If it is not so.
Then even ':' which is just adding to list head would be an O(n)
operation just because it should return brand new list with one elem
added. Or maybe functional approach uses pretty much different
complexity metric, there copying of some structure "list" for example
is just O(1)? If so then Q about compiler is still exists.

* * * * *

Tom Ellis:

Sounds like magic doesn't it :)

But no, there's no compiler magic, just an amazing datastructure.  The
caveat is that the complexity is amortised, not guaranteed for every
operation.  Have a look at the paper if you learn about how it works.  It's
linked from the Hackage docs.

   http://hackage.haskell.org/package/containers-0.2.0.1/docs/Data-Sequence.html


* * * * *

Asker:

Jake It would be great if you give some examples when find your
notebook :) And link to the book about pure functional data structures
which you are talking about.
Also If some "haskell.org" maintainers are here I'd like to recommend
them to pay more attention to optimality/performance questions.
Because almost first question which is apeared  in head of standart
C/C++ programmer is "Do I get same perfomance?" (even if he do not
need it).
Maybe some simple and cool PDF tutorial which describes why haskell
could be as fast as others will be great to have.

* * * * *

Richard A. O'Keefe:


> I am teaching myself haskell. The first impression is very good...
> Anyway I am trying to backport my algorithm written in C. The key to
> performance is to have ability to remove element from the end of a
> list in O(1).

You can't.  Not in *any* programming language.  That's because
lists are one of many possible implementations of the "sequence"
concept, and they are optimised to support some operations at
the expense of others.  At the beginning level, you should think
of all Haskell data structures as immutable; fixed; frozen;
forever unchanged.  You can't even remove an element from the
front of a Haskell list, at all.  All you can do is to forget
about the original list and concentrate on its tail.

> But the original haskell functions last and init are O(n).

Haskell lists are singly linked lists.  Even by going to
assembly code, you could not make these operations O(1)
without *using a different data structure*.

> My questions are:
> 1) Is last function is something like "black box" written in C++ which
> perform O(1)?

No.

> 2) Or will optimizer (llvm?) reduce init&last complexity to 1?

No.

> 3) Some people suggest to use sequences package, but still how do they
> implement O(1) init&last sequences equivalent in haskell?

Well, you could try reading Chris Okasaki's functional data
structures book.

There is a classic queue representation devised for Lisp
last century which represents
 <a,b,c,d,e>
by ([a,b],[e,d,c])
so that you can push and pop at either end.
When the end you are working on runs out, you
reverse the other end, e.g.,
  ([],[e,d,c]) -> ([c,d,e],[]).

That can give you a queue with *amortised* constant time.
(There is a technical issue which I'll avoid for now.)

But let's start at the beginning.
You have an interesting problem, P.
You have an algorithm for it, A, written in C.
You want an algorithm for it, H, written in Haskell.
Your idea is to make small local syntactic changes
to A to turn in into H.
That's probably going to fail, because C just
loves to smash things, and Haskell hates to.
Maybe you should be using quite a different approach,
one that would be literally unthinkable in C.
After all, being able to do things that are unthinkable
in C is one of the reasons for learning Haskell.

Why not tell us what problem P is?

* * * * *

Tony Morris:

data SnocList a = SnocList ([a] -> [a])

Inserts to the front and end in O(1).

### I consider the following conclusive

Edward Kmett:

Note: all of the options for playing with lists and queues and fingertrees come with trade-offs.

Finger trees give you O(log n) appends and random access, O(1) cons/uncons/snoc/unsnoc etc. but _cost you_ infinite lists.

Realtime queues give you the O(1) uncons/snoc. There are catenable output restricted deques that can preserve those and can upgrade you to O(1) append, but we've lost unsnoc and random access along the way.

Skew binary random access lists give you O(log n) drop and random access and O(1) cons/uncons, but lose the infinite lists, etc.

Tarjan and Mihaescu's deque may get you back worst-case bounds on more of the, but we still lose O(log n) random access and infinite lists.

Difference lists give you an O(1) append, but alternating between inspection and construction can hit your asymptotics.

Lists are used by default because they cleanly extend to the infinite cases, anything more clever necessarily loses some of that power.

## listen in Writer monad

```
20:26 < ifesdjee_> hey guys, could anyone point me to the place where I could read up on how `listen` of writer monad works?
20:26 < ifesdjee_> can't understand it from type signature, don't really know wether it does what i want..
20:30 < ReinH> :t listen
20:30 < lambdabot> MonadWriter w m => m a -> m (a, w)
20:31 < mm_freak_> ifesdjee_: try this: runWriterT (listen (tell "abc" >> tell "def") >>= liftIO . putStrLn . snd)
20:33 < mm_freak_> in any case 'listen' really just embeds a writer action and gives you access to what it produced
20:33 < ifesdjee_> most likely i misunderstood what happens in `listen`...
20:34 < ifesdjee_> i thought i could access current "state" of writer
20:34 < mm_freak_> remember that the embedded writer's log still becomes part of the overall log
20:34 < mm_freak_> execWriter (listen (tell "abc") >> tell "def") = "abcdef"
20:35 < mm_freak_> all you get is access to that "abc" from within the writer action
20:35 < ifesdjee_> yup, I see
20:35 < ifesdjee_> thank you a lot!
20:35 < mm_freak_> my pleasure
20:37 < mm_freak_> i wonder why there is no evalWriter*
20:37 < ifesdjee_> not sure, really
```

## Introduction and origination of free monads

```
21:32 < sclv> does anyone have a citation for the introduction of free monads?
21:33 < sclv> they’re so universally used in the literature nobody cites where they came from anymore
21:33 < sclv> in a computational context goes back to ’91 at least
21:40 < sclv> found it
21:40 < sclv> coequalizers and free triples, barr, 1970
```

http://link.springer.com/article/10.1007%2FBF01111838#page-1

Note: Seeing a paper on free monoids dating to 1972 by Eduardo J. Dubuc.

## Rank 2 types and type inference

```
03:13 < shachaf> dolio: Do you know what people mean when they say rank-2 types are inferrable?
03:14 < dolio> Not really. I've never taken the time to understand it.
03:16 < dolio> One reading makes no sense, I think. Because rank-2 is sufficient to lack principal types, isn't it?
03:17 < dolio> Or perhaps it isn't....
03:17 < shachaf> Well, you can encode existentials.
03:17 < dolio> Can you?
03:17 < dolio> forall r. (forall a. a -> r) -> r
03:17 < dolio> I guess that's rank-2.
03:18 < shachaf> You can give rank-2 types to expressions like (\x -> x x)
03:18 < shachaf> What type do you pick for x?
03:19 < dolio> forall a. a -> β
03:19 < dolio> Presumably.
03:20 < shachaf> Does β mean something special here?
03:20 < dolio> It's still open.
03:20 < dolio> Greek for unification variables.
03:21 < shachaf> OK, but what type do you infer for the whole thing?03:21 < dolio> forall r. (forall a. a -> r) -> r
03:23 < dolio> (\f -> f 6) : forall r. (Int -> r) -> r
03:23 < dolio> Is that a principal type?
03:23 < shachaf> Do you allow type classes?
03:24 < dolio> People who say rank-2 is decidable certainly shouldn't be thinking about type classes.
03:24 < shachaf> I guess with impredicativity the type you gave works... Well, does it?
03:25 < dolio> Maybe rank-2 is sufficient to eliminate all ambiguities.
03:25 < dolio> Like, one common example is: [id]
03:25 < dolio> Is that forall a. [a -> a] or [forall a. a -> a]
03:25 < dolio> But, we're not talking about Haskell, we're talking about something like system f.
03:26 < dolio> So you'd have to encode.
03:26 < dolio> And: (forall r. ((forall a. a -> a) -> r -> r) -> r -> r) is rank-3.
03:27 < shachaf> I guess...
03:27 < dolio> If I had to guess, that's what the answer is.
```

- Practical type inference for arbitrary-rank types - Peyton Jones, Vytinotis, Weirich, Shields

- http://stackoverflow.com/questions/9259921/haskell-existential-quantification-in-detail

- http://en.wikibooks.org/wiki/Haskell/Polymorphism