Hi Taral, Eugene,
[Taral]
Perhaps I don't understand Agda very well, but I don't see
parametricity here. For one, there's no attempt to prove that:
forall (P Q : forall a, a - a), P = Q.
[Eugene]
Under parametricity, I mean the Reynolds Abstraction Theorem, from
which free theorems follow.
Hi Duncan and all,
On Wed, Mar 25, 2009 at 3:52 AM, Duncan Coutts
duncan.cou...@worc.ox.ac.uk wrote:
On Mon, 2009-03-23 at 08:11 -0400, Xiao-Yong Jin wrote:
tryArith :: a - Either ArithException a
tryArith = unsafePerformIO . try . evaluate
You must not do this. It breaks the semantics of
On Thu, Mar 26, 2009 at 2:40 AM, Duncan Coutts
duncan.cou...@worc.ox.ac.uk wrote:
I was not being precise when I said tests for _|_. As you point out,
the semantics of imprecise exceptions distinguishes exceptions from
bottom, however pure code cannot make that distinction and so that's why
I
2009/3/24 Peter Verswyvelen bugf...@gmail.com:
This strategy is doomed to failure, unfortunately.
So it is the good strategy, because Haskell's slogan is avoid success at
all cost :-)
IN THE YEAR 1987, WAR WAS BEGINNING
BIG, IMPERATIVE SOFTWARE BEHEMOTHS CLASHED IN A STATE OF IMPURITY
Hi all,
On Tue, Feb 10, 2009 at 3:26 PM, Malcolm Wallace
malcolm.wall...@cs.york.ac.uk wrote:
If you have ideas for student projects that you think would benefit the
Haskell community, now is the time to start discussing them on mailing
lists of your choice. We especially encourage students
Hi Lennart,
On Sat, Jan 24, 2009 at 10:47 PM, Lennart Augustsson
lenn...@augustsson.net wrote:
You can dream up any semantics you like about bottom, like it has to
be () for the unit type.
But it's simply not true. I suggest you do some cursory study of
denotational semantics and domain
On Sun, Jan 18, 2009 at 5:48 PM, rocon...@theorem.ca wrote:
I noticed the Bool datatype isn't well documented. Since Bool is not a
common English word, I figured it could use some haddock to help clarify it
for newcomers.
-- |The Bool datatype is named after George Boole (1815-1864).
--
On Tue, Dec 23, 2008 at 2:52 AM, Antoine Latter aslat...@gmail.com wrote:
Although I still had to use my own because I wanted a MonadPlus
instance. I would offer a patch, but since there's more than one
useful MonadPlus instance for MaybeT it probably still wouldn't be
right for everyone.
On Tue, Dec 23, 2008 at 8:05 PM, Benja Fallenstein
benja.fallenst...@gmail.com wrote:
Umh, there is a MonadPlus instance in the package?
Ah: ...in the version Cale uploaded two days ago, not in the previous
version. Sorrynevermindisee :)
- Benja
Hi all,
On Wed, Nov 12, 2008 at 2:09 PM, Lennart Augustsson
[EMAIL PROTECTED]wrote:
You can't write a straightforward dynamic semantics (in, say,
denotational style) for Haskell.
The problem is that with type classes you need to know the types
compute the values.
...
It's possible that
Hi Maurício,
I've got one thing to add to the replies so far:
On Wed, Aug 27, 2008 at 8:23 PM, Maurício [EMAIL PROTECTED] wrote:
What does '~' mean in Haskell? I
read in haskell.org/haskellwiki/Keywords
that (...) Matching the pattern ~pat
against a value always suceeds, and
matching will
Hi,
On Sun, Aug 10, 2008 at 8:57 PM, Michael Feathers
[EMAIL PROTECTED] wrote:
If I have, say, a function f :: a - a - a - a - b it would be nice to be
able to just:
unlistN 4 f [1..4]
It indeed doesn't work like this; there's more than one way to do
something *like* this, if you really want
Hi Pieter,
2008/8/5 Pieter Laeremans [EMAIL PROTECTED]:
But the sphinx indexer complains that the xml isn't valid. When I look at
the errors this seems due to some documents containing not well formed
html.
If you need to cope with non-well-formed HTML, try HTML Tidy:
Hi Edsko,
On Jan 22, 2008 7:34 PM, Edsko de Vries [EMAIL PROTECTED] wrote:
Is there a nice way to pretty-print such an expression with the minimal
number of brackets? I can come up with something, but I'm sure somebody
thought hard about this problem before and came up with a really nice
Hi Christian,
On Jan 21, 2008 10:57 AM, Christian Maeder [EMAIL PROTECTED] wrote:
Thanks for pointing out this proposal. The actual problem is mkGraph
that needs all the many edges created beforehand (that's what I wanted
to avoid).
Well, uh, at the risk of being obvious, if you can avoid
Hi Christian,
On Jan 18, 2008 1:55 PM, Christian Maeder [EMAIL PROTECTED] wrote:
data CGraph a b = CGraph [a] (a - a - b)
Can I define an instance for the fgl Graph class?
I had no idea how to define empty (except using undefined).
Well, presumably the function does not need to be defined
Hi,
On Jan 19, 2008 6:05 PM, Thomas Hartman [EMAIL PROTECTED] wrote:
Do you just assume that every two nodes have an edge between them [...]?
Since that's what complete graph means, I assume so =-)
- Benja
___
Haskell-Cafe mailing list
Hi Mitar,
On Jan 10, 2008 9:22 AM, Mitar [EMAIL PROTECTED] wrote:
I understand that proper mathematical behavior would be that as 0/0 is
mathematically undefined that 0/0 cannot be even compared to 1.
My understanding is that common mathematical practice is that
comparing an undefined value to
On Jan 10, 2008 11:54 PM, Luke Palmer [EMAIL PROTECTED] wrote:
Can someone explain what the heck is going on here?
Evaluating (const EXPR) creates a closure object with a pointer to
'const' and a pointer to the EXPR thunk. Call this closure object C.
Evaluating (C undefined) calls 'const' with
Hi Yitz,
On Jan 2, 2008 10:34 AM, Yitzchak Gale [EMAIL PROTECTED] wrote:
No, only countably many. By the type expression Integer - Integer
we mean all Haskell functions mapping Integers to Integers.
There are only countably many of those.
...
But that was not the context in this thread. The
On Dec 31, 2007 7:17 AM, [EMAIL PROTECTED] wrote:
This declaration states that there's a bijection between the elements of
Foo and the elements of 2^Foo, which by Cantor's diagonal theorem cannot
be true for any set. That's because we only allow computable functions,
Nit the nit: Or (more
Hi Cristian,
On Dec 30, 2007 6:10 PM, Cristian Baboi [EMAIL PROTECTED] wrote:
What I don't get it :
(s a1 a2 ... at) must be the value of A in the semantic domain. Let call
that value a.
Then how can one know if a was built with (s a1 a2 ... at) and not with
(egg b1 b2) ?
Because the
On Dec 28, 2007 5:14 PM, Mike Haskel [EMAIL PROTECTED] wrote:
You can define Show as a data type, rather than a type class:
type Show a = Either (a - String) (Int - a - String - String)
...
The constructors for Show make explicit the two ways to define an
instance. This technique also has
Hi all,
On Dec 28, 2007 12:38 PM, Andrew Coppin [EMAIL PROTECTED] wrote:
For joining you probably want some combination of intersperse and
concat, e.g.
unlines = concat . intersperse \n
And that's what we have :-)
Data.List.intercalate :: [a] - [[a]] - [a]
Data.List.intercalate x = concat
On Dec 28, 2007 3:55 PM, David Roundy [EMAIL PROTECTED] wrote:
On Dec 28, 2007 9:51 AM, Benja Fallenstein [EMAIL PROTECTED] wrote:
If you use intercalate to join, I would presume that you would want to
use an inverse of it to split. I'd write it like this:
Of course, there is no inverse
On Dec 28, 2007 4:24 PM, Benja Fallenstein [EMAIL PROTECTED] wrote:
Right; I misspoke. What I meant was that you would want a split such that
intercalate a (split a xs) = a
for finite, total (a,xs) (and, since it's achievable, even for
infinite xs). Of course, (split a xs = [xs
On Dec 23, 2007 1:44 PM, Isaac Dupree [EMAIL PROTECTED] wrote:
parseHeader3 :: BS.ByteString - Maybe (Int, Int)
parseHeader3 bs = do
(x, rest) - BS.readInt $ BS.dropWhile (not . isDigit) bs
(y, _) - BS.readInt $ BS.dropWhile (not . isDigit) rest
return (x, y)
But that version still
Hi Neil,
On Dec 26, 2007 7:16 PM, Neil Mitchell [EMAIL PROTECTED] wrote:
Given the code:
fred = 2 + 2
bob = fred + fred
In a Haskell implementation fred would be evaluated once to 4, then
used twice. The 2+2 would only happen once (ignore defaulting and
overloaded numerics for now).
Is
Hi Cristian,
On Dec 18, 2007 10:53 AM, Cristian Baboi [EMAIL PROTECTED] wrote:
- the lambda expressions can be written (input) but cannot be printed
(output)
Yes, since two different lambda expressions can denote the same function.
I just want the sistem to be able to print one of these
Hi Henning,
On Dec 18, 2007 3:53 PM, Henning Thielemann
[EMAIL PROTECTED] wrote:
Since this was discussed already here, I summed it up in:
http://www.haskell.org/haskellwiki/Show_instance_for_functions
I find the discussion under theoretical answer unsatisfying. The
property that a Show
On Dec 18, 2007 4:50 PM, Benja Fallenstein [EMAIL PROTECTED] wrote:
Further, even with extensionality, we can (with compiler support) in
principle have Show instances other than enumerating the graph.
Now that I said it, I'm starting to doubt we even need compiler
support beyond what we have
Hi Henning,
On Dec 18, 2007 5:17 PM, Henning Thielemann
[EMAIL PROTECTED] wrote:
The mathematical definition of function I know of, says that functions
are special relations, and relations are sets of pairs. Their is nothing
about intension.
That's the standard definition in set theory, but
Hi Paul,
On Dec 18, 2007 5:18 PM, Paul Hudak [EMAIL PROTECTED] wrote:
If the semantics of a language says that a function f is equivalent to a
function g, but there is a function h such that h(f) is not equivalent to
h(g), then h cannot be a function.
Sure.
Therefore that language cannot
On Dec 18, 2007 6:01 PM, Paul Hudak [EMAIL PROTECTED] wrote:
Well, my caveat was that the Haskell designers wanted it this way. So
you are essentially rejecting my caveat, rather than creating a new one.
:-)
I mean, I reject the answer They wanted it this way because I think
the answer should
Hi all,
Below is a program that implements Show for functions whose type is
composed of only (-) and type variables (or, more precisely, of (-)
and (State Int Term), but any type composed of (-) and type variables
can obviously be specialized to that).
(-fglasgow-exts is needed only for the
Hi Paul,
On Dec 19, 2007 6:54 AM, Paul Hudak [EMAIL PROTECTED] wrote:
Your version of the answer is in fact correct, but is just an elaboration
of the original one.
So, I don't see what your point is...
Ok, sorry, I'll try again... I'm trying to say that in my opinion,
it's important to
Hi Corey,
On Dec 14, 2007 8:44 PM, Corey O'Connor [EMAIL PROTECTED] wrote:
The reason I find all this odd is because I'm not sure how the type
class Functor relates to the category theory concept of a functor. How
does declaring a type constructor to be an instance of the Functor
class relate
On Dec 15, 2007 3:44 AM, Benja Fallenstein [EMAIL PROTECTED] wrote:
Hmmm. Something about that ticks off my don't play fast and loose
with bottom detector.
I should add that I do think you're correct if you ignore the
existence of bottom, and I'm pretty sure that you're correct if you
allow
On Dec 14, 2007 10:38 AM, Bulat Ziganshin [EMAIL PROTECTED] wrote:
hUnsafeGetContentsDontUseThisUnlessYouHaveSpentThreeMonthsLearningGHCsExecutionSemanticsOrYouWillRegretIt
i have more advanced proposal - we should include in its name whole
paper on its semantics so anyone using it will be
On Dec 14, 2007 9:29 AM, Henning Thielemann
[EMAIL PROTECTED] wrote:
I remember there was a discussion about how to implement full 'wc' in an
elegant but maximally lazy form, that is counting bytes, words and lines
in one go. Did someone have a nice idea of how to compose the three
counters
Hi Thomas,
On Dec 12, 2007 5:31 PM, Thomas Hartman [EMAIL PROTECTED] wrote:
(solution involves building an accum list of (average,listLength) tuples.
again you can't do a naive fold due to stack overflow, but in this case even
strict foldl' from data.list isn't strict enough, I had to define
On Dec 12, 2007 9:58 PM, Don Stewart [EMAIL PROTECTED] wrote:
And no need to even use custom ones, just use the library strict pairs,
http://hackage.haskell.org/packages/archive/strict/0.2/doc/html/Data-Strict-Tuple.html
Oh, good! :)
'nother Haskell lesson learned. Thanks,
- Benja
Another version of detab:
main = interact $ perLine $ concat . snd. mapAccumL f 0 where
f tab '\t' = (0, replicate (4-tab) ' ')
f tab char = ((tab+1) `mod` 4, [char])
- Benja
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
On Dec 13, 2007 2:20 AM, Benja Fallenstein [EMAIL PROTECTED] wrote:
Another version of detab:
main = interact $ perLine $ concat . snd. mapAccumL f 0 where
f tab '\t' = (0, replicate (4-tab) ' ')
f tab char = ((tab+1) `mod` 4, [char])
Although on reflection, I think I might like
On Dec 13, 2007 2:28 AM, Benja Fallenstein [EMAIL PROTECTED] wrote:
Although on reflection, I think I might like the following compromise
with Tillmann's version best:
main = interact $ perLine $ detab 0 where
detab tab ('\t':cs) = replicate (4-tab) ' ' ++ detab 0 cs
Hi Conal,
On Dec 9, 2007 6:09 PM, Conal Elliott [EMAIL PROTECTED] wrote:
readIVar' :: IVar a - a
readIVar' = unsafePerformIO . readIVar
so, we do not need readIVar'. it could be a nice addition to the
libraries, maybe as unsafeReadIVar or unsafeReadMVar.
The same argument applies any to
On Dec 7, 2007 6:57 PM, Peter Padawitz [EMAIL PROTECTED] wrote:
Jules Bean wrote:
Peter Padawitz wrote:
Cause I don't see why the instantiation conflicts pointed out by
others would vanish then.
They would.
If it's really true that there is only one possible choice of b,c,d
for
Hi Paolo,
On Dec 5, 2007 2:09 PM, Paulo J. Matos [EMAIL PROTECTED] wrote:
I'm glad that my initial post generated such an interesting discussion
but I'm still not understanding why the first version of findAllPath
seems to be computing the whole list even when I just request the
head, while
On Dec 5, 2007 5:40 PM, Paulo J. Matos [EMAIL PROTECTED] wrote:
Oh, but lfpaths is not nothing so that means that isNothing rtpaths
shouldn't be evaluated, right?
You're right, and I was stupid not to think about that case. Since
Luke already gave an in-depth analysis I'll be quiet now :-)
-
Hi Radosław,
You should be able to write this with MaybeT as follows:
getStrip :: IO ( Maybe String )
getStrip = runMaybeT $ do
pageContent - liftIO $ downloadFile mainPageAddress
let x = patternForStrip pageContent
print x
z - x
liftIO $ downloadFile $ mainPageAddress ++ z
On 11/18/07, Benja Fallenstein [EMAIL PROTECTED] wrote:
Hi Radosław,
You should be able to write this with MaybeT as follows:
Correction, sorry. The code in my original mail doesn't take care of
converting the 'Maybe's returned by the functions you're calling into
'MaybeT's.
The following
2007/7/19, Jason Dagit [EMAIL PROTECTED]:
I prefer,
allEqual [] = True
allEqual xs = foldl1 (==) xs
But, unfortunately, it's not a one liner like yours (unless you allow
allEqual [] = undefined).
Oh and silly me, that only works for [Bool].
My natural instinct is,
allEqual [] = True
2007/7/20, Bulat Ziganshin [EMAIL PROTECTED]:
allEqual [] = True
allEqual (x:xs) = all (== x) xs
with the same caveat about allEqual [] as in your case.
allEqual xs = all (== head xs) xs
Rght. Not evaluated in the edge case, because xs is empty. Didn't
think of that, nice :-)
-
Hi Peter,
2007/7/12, peterv [EMAIL PROTECTED]:
Q1) Is it possible to treat a tuple of N elements in a generic way? So
instead of writing functions like lift1 e1, lift2 (e1,e2), lift3 (e1,e2,e3)
just one function liftN that works on tuples of any length?
Q2) (Maybe related to Q1) Can I convert
2007/7/12, peterv [EMAIL PROTECTED]:
Amazing, so simple it is, Yoda would say ;)
I did not realize one could perform partial application on types when
declaring instances (I mean not specifying the type of Vector2 in instance
Vector Vector2).
You ought to meditate on the type class 'Monad,'
Hi Adrian,
2007/7/8, Adrian Hey [EMAIL PROTECTED]:
So it seems ghc doesn't like kinds (* - *) either :-(
Actually, AFAICT the problem seems to be with Data.Typeable
itself rather than ghc. There is no proper TypeRep for
(ListGT map k a) because map is not a type.
Have you tried using
Hi,
2007/7/1, Bulat Ziganshin [EMAIL PROTECTED]:
aa - unsafeRead a5 0
return $! aa
bb - unsafeRead a5 1
return $! bb
If this is a useful pattern, would it make sense to have a function to
encapsulate it?
mseq :: Monad m = m a - m a
mseq m = m = (return $!)
- Benja
Hi Ken,
2007/7/1, Ken Takusagawa [EMAIL PROTECTED]:
I'd like to have a state monad with the feature that I can somehow
annotate using the type system that some functions are only going to
read the state and not modify it.
I would suggest declaring a MonadReader instance for State, and
writing
Hi Hugh,
2007/6/25, Donald Bruce Stewart [EMAIL PROTECTED]:
hughperkins:
Just noticed that all my responses have been going only to
Neil, not to the group.
Anyway, the jist of our conversation was that it's not
possible to create arbitrary datatypes/constructors from
Hi Peter,
2007/6/25, peterv [EMAIL PROTECTED]:
I'm baffled. So using the Arrow abstraction (which I don't know yet) would
solve this problem? How can (perfectActionB x) be checked with without ever
executing performActionA which evaluates to x? This can only be done when x
is a constant
Hi all,
The scrap your boilerplate with class sytstem [1] has two big
advantages over the plain SYB system from Data.Generics, IMHO: One, it
lets you declare an 'open' generic function as a type class, to which
new cases can be added by adding new instances (emphasized in the
paper); and two, it
2007/6/25, Michael T. Richter [EMAIL PROTECTED]:
OK, just to prevent this getting side-tracked: I'm absolutely
uninterested in the results of performActionA before determining if
performActionB is permitted/possible/whatever. Think more in terms of
security permissions or resource
2007/6/22, Hugo Pacheco [EMAIL PROTECTED]:
class Functor f = C f a b | f a - b where
ftest :: f a - b
I want to write some function
test :: (C f a b) = (a - b)
test = ftest . undefined
I'm not sure whether this is what you want, but the obvious way to
make this type-check would seem to be
Hi,
I can't figure out why the following program compiles with this
instance declaration
instance Size (Maybe [a]) = Size [a] where size x = size (foo x)
but has GHC loop forever with this one:
instance (Foo a b, Size b) = Size a where size x = size (foo x)
Anybody here know?
Thanks,
-
Hi Tony,
2007/6/16, Tony Morris [EMAIL PROTECTED]:
I was told on #haskell (IRC) the other day that it is possible to write
a general memoisation table using IORef and unsafePerformIO. I can't
think of how this can be achieved without writing to a file, since a
function cannot hold state between
Hi all,
We've had a discussion on #haskell about how we can make a function
that reads in serialized values of an open data type, such as
class (Show a, Read a) = MyClass a where
typeTag :: a - String
... operations on the open data type...
data Obj = forall a. MyClass a = Obj { unObj ::
Hi Isaac,
2007/6/13, Isaac Dupree [EMAIL PROTECTED]:
Since Show instances can overlap (e.g. (show (1::Int)) == (show
(1::Integer))), we need to tag with the type.
Indeed. But that's the easy part :-)
Reminds me of Typeable.
Since GHC lets us derive Typeable with a guarantee of different
Hi Bulat,
2007/6/13, Bulat Ziganshin [EMAIL PROTECTED]:
We've had a discussion on #haskell about how we can make a function
that reads in serialized values of an open data type, such as
look at Data.Generics.Text which may be implements exactly what you
need
Unfortunately not.
2007/6/14, Stefan O'Rear [EMAIL PROTECTED]:
On Wed, Jun 13, 2007 at 05:12:25PM +0300, Benja Fallenstein wrote:
However, this is still kind of boring. Is there a better way? If not,
would it be a good idea to have compiler support for building this
kind of type table?
The compiler does build
2007/5/12, Derek Elkins [EMAIL PROTECTED]:
In Haskell codata and data coincide, but if you want consistency, that cannot be
the case.
For fun and to see what you have to avoid, here's the proof of Curry's
paradox, using weird infinite data types. We'll construct an
expression that inhabits any
2007/5/13, Benja Fallenstein [EMAIL PROTECTED]:
Modulo the constructor and destructor invocation, this is just the
familiar non-terminating ((\x - x x) (\x - x x)), of course.
The same technique also gives us
data Y a = Y (Y a - a)
y :: (a - a) - a
y f = (\(Y x) - f $ x $ Y x) $ Y $ (\(Y x
Adding some thoughts to what David said (although I don't understand
the issues deeply enough to be sure that these ideas don't lead to
ugly things like paradoxes)--
2007/5/10, Gaal Yahas [EMAIL PROTECTED]:
Since the empty list inhabits the type [b], this theorem is trivially
a tautology, so
Hi all,
Is there a way to achieve the effect of soft references in GHC? Or if
not, is there any hope that they will be implemented in the future?
(Soft references are like weak references, except that they are only
reclaimed by the garbage collector if memory is short.)
I'm building a
Hi,
(new here)
2007/3/25, Jacques Carette [EMAIL PROTECTED]:
Some classes would become even more important: monoid, groupoid,
semi-group, loop (semi-group with identity), etc. But all of those are,
to the average programmer (and many a mathematician), just as scary as
Monad.
Of course, when
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