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Today's Topics:
1. Re: Help on first program (John M. Dlugosz)
2. Re: Help on first program (Brandon Allbery)
3. Why is this not a "category"? (John M. Dlugosz)
4. Re: Why is this not a "category"? (Arjun Comar)
5. understanding MTL (Dennis Raddle)
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Message: 1
Date: Fri, 28 Mar 2014 23:13:19 -0500
From: "John M. Dlugosz" <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Help on first program
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
On 3/28/2014 6:56 PM, Brandon Allbery wrote:
> It uses all of them to get the type, yes. And the more specific pattern must
> come first;
> the first one always matches in this case because `n` doesn't give it any way
> not to
> match. If you had warnings enabled, the compiler should have warned you that
> the second
> form wouldn't be matched (although you may also need optimization turned on).
I was wondering about that. I'll look for warning flags. I was loading into
GHCi rather
than running the compiler from the command line.
>
> The compiler doesn't see the different implementations as independent, and in
> fact doesn't
> see multiple implementations of the function at all at type resolution time;
> it's
> translated to a single function applying `case` to the parameters to
> determine which
> clause of the body to evaluate.
Can they be spread out among different source files or be discontiguous within
one file?
------------------------------
Message: 2
Date: Sat, 29 Mar 2014 00:22:31 -0400
From: Brandon Allbery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Help on first program
Message-ID:
<CAKFCL4W0bQhC_ikTi5Y7OuoZ0+vVgJ=4avnFxMbssg=vhc+...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Sat, Mar 29, 2014 at 12:13 AM, John M. Dlugosz
<[email protected]>wrote:
> On 3/28/2014 6:56 PM, Brandon Allbery wrote:
>
>> The compiler doesn't see the different implementations as independent,
>> and in fact doesn't
>>
> see multiple implementations of the function at all at type resolution
>> time; it's
>> translated to a single function applying `case` to the parameters to
>> determine which
>> clause of the body to evaluate.
>>
>
> Can they be spread out among different source files or be discontiguous
> within one file?
Neither one; they must be together in the same file with nothing
intervening, since they get rewritten into a single function. They also
must all have the same number of parameters, even if one of them could take
advantage of eta reduction (see
http://www.haskell.org/haskellwiki/Eta_conversion), since the combination
into a single function uses a common set of parameters.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 3
Date: Sat, 29 Mar 2014 00:18:46 -0500
From: "John M. Dlugosz" <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Why is this not a "category"?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
on
https://en.wikibooks.org/w/index.php?title=Haskell/Category_theory&stable=0#Hask.2C_the_Haskell_category
the second exercise in the box (see illustration there) asks
"(Harder.) If we add another morphism to the above example, it fails to be a
category.
Why? Hint: think about associativity of the composition operation."
There are no answers-to-exercises. Can someone explain to me why adding
another function
with the same type causes the Haskell type system to no longer form the Hask
category?
(scratching head)
------------------------------
Message: 4
Date: Sat, 29 Mar 2014 01:47:01 -0400
From: Arjun Comar <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Why is this not a "category"?
Message-ID:
<CADjRcrUUugZPzvUeyUZUT=wq5pjv3kzu0344esbubq54hk-...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
You're misreading the question. It's asking you to show that the category
induced by the <= relation fails associativity if you add an extra
morphism.
On Mar 29, 2014 1:19 AM, "John M. Dlugosz" <[email protected]> wrote:
> on https://en.wikibooks.org/w/index.php?title=Haskell/
> Category_theory&stable=0#Hask.2C_the_Haskell_category
>
> the second exercise in the box (see illustration there) asks
> "(Harder.) If we add another morphism to the above example, it fails to be
> a category. Why? Hint: think about associativity of the composition
> operation."
>
> There are no answers-to-exercises. Can someone explain to me why adding
> another function with the same type causes the Haskell type system to no
> longer form the Hask category?
>
> (scratching head)
>
> _______________________________________________
> Beginners mailing list
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Message: 5
Date: Sat, 29 Mar 2014 03:32:09 -0700
From: Dennis Raddle <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] understanding MTL
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I had an interaction on #haskell today which left me utterly confused but
desiring to understand what happened. I was talking about a program in
which I was using stacks of monad transformers, inspiration taken from the
paper "Monad Transformers: Step by Step". So for example I wanted to
combine state and error handling in a Monadic type called "Er".
data ErState = ErState StdGen
newtype Er a = Er { runEr :: ErrorT String (State ErState) a }
deriving(Monad, MonadError String, MonadState ErState)
Note that ErState holds a random generator. I had the idea to make a
typeclass of monadic types that hold random generators because I'll be
using them in other types all over my application. Something like
class Monad m => RandomMonad m where
getGen :: m StdGen
putGen :: StdGen -> m ()
Then I can write functions like this:
myRandoms :: (Random a, RandomMonad m) => m [a]
myRandoms = do
g <- getGen
let (g1, g2) = split g
values = randoms g1
putGen g2
return values
So then mm_freak on #haskell noticed that I had written a function with a
signature like
process :: Int -> Er Int
He said, no no no functions should be agnostic with regard to data
structure. It should look something like
process :: (RandomMonad m, MonadError m, MonadState m) => Int -> m Int
He then said I don't want to put StateT and ErrorT in the same newtype
declaration, so it would be something like
newtype Er m a = Er { runEr :: StateT ErState m a }
On this point I'm confused. I don't know how to get from a line like the
above to eventually making a type that combines error handling and state.
Furthermore I need to define instances for things like MonadError. He got
me started with the line
instance MonadError e m => MonadError e (Er m) where
... define throwError and catchError which I have no idea how to do ...
So where we left it, I was confused and didn't understand anything.
Rather than just getting the answers, I want to try to understand this, but
I feel like I must need more background. This must be the kind of code that
is in the monad transformer library. Is there a guide to understanding that
somewhere?
D
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