(software reuse!) is obvious, then.
2. Software for a banking-house that handles all the financial
transactions just cannot be shut down for bug fixing or rewrites. The
same goes for lambdabot :)
Regards,
apfelmus
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Can you add the mailing list to GMANE?
Right q - return p
guard (p 5)
return (p-5)
One last thing is to eliminate fromJust:
f x
| (interesting things here)
should be syntactic sugar for
f x = fromJust $
| (interesting things here)
Regards,
apfelmus
follow-ups (possibly indented). This expends of any
unnecessary ;
do/choose
| Right q ~ p
val - lookup key
v - do/choose
| Left v ~ val
return v
| Right v - val
return v
return (v+1)
| Left q ~ p
Regards,
apfelmus
conditions b to filter things out.
Regards,
apfelmus
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startup which is probably what you want. If not, then you just have to
float (\s -) out of unsafePerformIO:
getString = \s - unsafePerformIO $ do ... return $ fromJust' ...
Of course, you can also integrate the initialization into (getString).
Regards,
apfelmus
I think I remember that some program was used to generate either the
Hugs or GHCi ascii logo that appears when you fire up the interpreter.
While not being the same, there's a similar program in the Hugs demos
directory:
/usr/lib/hugs/demos/Say.hs
Say putStr $ say Hugs
H H U U
) and not that of (foldl) as
the latter can be recovered from the former but not the other way round.
Regards,
apfelmus
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out is evaluated strictly, only that the pair
constructor is matched by a refutable pattern. In other words, is there
an example where one would prefer (foo (a,b) = ...) over (foo ~(a,b) =
...) for reasons of time and space? If not, then I think it's a bug.
Regards,
apfelmus
such a list.
Intentions: I need views for ...
* pattern matching on abstract data types (John Meacham, apfelmus)
examples:
- Okasaki: Breath first numbering - lessons from a small exercise
in graph theory
view Queue a = Empty | Cons a (Queue a)
- ByteStrings
view
. The countdown problem.
Journal of Functional Programming, 12(6):609-616, November 2002
http://www.cs.nott.ac.uk/~gmh/countdown.pdf
Regards,
apfelmus
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of Programming.
Prentice Hall, 1996.
but it's not introductory in nature. It seems to be out of print, is
there any way to get a (perhaps electronic) copy?
Regards,
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, the SML version uses Vectors, polymorphic vectors, immutable
sequences with constant-time access. I wonder how immutable these are.
I suspect that changing elements does updates in place with history
tracking?
Regards,
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to hit the bottom. Putting both
together, we see that we have to pay O(n + k*log n) steps to build the
expression and to fetch the first k elements.
Making this argument rigorous with a suitable debit invariant is left to
the attentive reader :)
Regards,
apfelmus
Regards,
apfelmus
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, fuzzy thing sequence $ words Warm, fuzzy thing
[M,o,n,a,d]
Oh, this output is unexpectedly cold and clear.
Regards,
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apfelmus wrote:
Steffen Mazanek wrote:
From my understanding for small k's lazy
evaluation already does the trick for the naive quicksort
algorithm (quicksort smaller ++ [x] ++ quicksort greater),
doesn't it? Is there a search algorithm that makes better
use of lazy evaluation out of the box
with lazy evaluation. Isn't it great that finding the k-th
minimum is not only an adaption of quicksort but can readily be obtained
from it by _composing_ it with (!! k)?
Regards,
apfelmus
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is a monad that generates code without any execution
taking place. The execution part is already handled by runCodeGen.
Having liftIO means that arbitrary Haskell programs can be intertwined
with assembly generation and I doubt that you want that.
Regards,
apfelmus
invariants on one and the same map type. In other words, the different
Ord instances for A will mess up the invariants of the Map A a
implementation and will eventually even lead to pattern match failures.
Being able to mess up invariants by class export/import is quite dangerous.
Regards,
apfelmus
(sounds silly
for mail-only but is extremely useful for read-through-gmane).
Regards,
apfelmus
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. But for
CSE, we have to carry the collection around anyway.
I don't know any references for that method since I came up with it
myself and haven't searched around yet. Any pointers?
Regards,
apfelmus
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type signatures
cata :: Fixpoint f t = (f s - s) - t - s
size :: (Fixpoint f t, Foldable f) = t - Int
Regards,
apfelmus
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or multi parameter type classes exist.
Regards,
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It's much shorter and should run in constant memory as well.
Regards,
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Christian Maeder wrote:
good bug! -O or -O2 is irrelevant but it works if compiled with -fvia-C
You (or someone else) should add it to
http://hackage.haskell.org/trac/ghc
I guess that this is related to
http://thread.gmane.org/gmane.comp.lang.haskell.cafe/31675
Regards,
apfelmus
/gmane.comp.lang.haskell.general/15471
In other words, the main problem of all those superclass/explicit
import/export proposals is that there are no proofs of the fact that
they only allow well-defined programs. The homework isn't done yet,
discussing adoption is too early.
Regards,
apfelmus
[] where
fmap = fmapDefault
fmap is already written for you, the instance declaration is only
boilerplate. I first saw this in Data.Traversable .
Regards,
apfelmus
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Right q - return p
guard (p 5)
return (p-5)
One last thing is to eliminate fromJust:
f x
| (interesting things here)
should be syntactic sugar for
f x = fromJust $
| (interesting things here)
Regards,
apfelmus
fixity levels as fixed points of
continuous functionals!
Regards,
apfelmus
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fixity levels are useful, too. A further
step to complex numbers is not advised because those cannot be ordered.
But ordering of the computable reals is not computable. So it could
cause the compiler to loop during parsing. :)
Actually, that's one of the use cases ;)
Regards,
apfelmus
are to be allowed in pattern
matching.
Regards,
apfelmus
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be
r - lookup m x
Regards,
apfelmus
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apfelmus suggested to use '=' for this purpose, so that,
wherever monadic generators
are permitted
pattern = expr ~~ pattern - return expr
It was to late when i realized that = is already used as smaller than
or equal to :)
Obviously, the difference between the pattern guard
the limits of Haskell syntax.
True. Compared to Template Haskell, a preprocessor allows syntactic
extensions but is weak at type correctness.
Regards,
apfelmus
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the join.
In any case, I'm *strongly against* further syntactic sugar for monads,
including #1518. The more tiresome monads are, the more incentive you
have to avoid them.
Regards,
apfelmus
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apfelmus wrote:
In the end, I think that applicatively used monads are the wrong
abstraction.
Simon Peyton-Jones wrote:
Can you be more explicit? Monadic code is often over-linearised.
I want to generate fresh names, say, and suddenly I have to name
sub-expressions. Not all sub-expressions
, they have to stand some test of time before Haskell' can pick one
of the two (probably the latter).
Regards,
apfelmus
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Iavor Diatchki wrote:
apfelmus wrote:
fundeps are too tricky to get powerful and sound at the same time.
I am not aware of any soundness problems related to functional
dependencies---could you give an example?
http://hackage.haskell.org/trac/haskell-prime/wiki/FunctionalDependencies
are not reported early via the consistency
condition but late when actually constructing terms. The consistency
condition should be enough for soundness (no inconsistent axioms
accepted), but I didn't dwell enough into FD to know such things for sure.
Regards,
apfelmus
) - a - b
for the /identity functor/. In other words, the current ($) and (.) are
just special cases of the general fmap . Unfortunately, the identity
functor currently can't be overloaded, although I think it would be
unambiguous.
Regards,
apfelmus
for all eternity :)
Regards,
apfelmus
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~) where
(.) = ()
Regards,
apfelmus
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Stefan O'Rear wrote:
apfelmus wrote:
My assumption is that we have an equivalence
forall a,b . m (a - m b) ~ (a - m b)
because any side effect executed by the extra m on the outside can well
be delayed until we are supplied a value a. Well, at least when all
arguments are fully applied
,
apfelmus
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Justin Bailey wrote:
apfelmus wrote:
Extracting the head and tail of ss with a let statement could lead to
a huge unevaluated expression like
rest = tail (tail (tail (...)))
Even though they are probably forced, would breaking the head and tail
apart via pattern-matching or a case
as an
intermediate result in the translation for empty Q.
Regards,
apfelmus
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(\new old - new) 'a' 1 (fromList [('a',2),('b',1)])
do? I can't yield
fromList [('a',1),('b',1)]
since 1 has two keys now.
Regards,
apfelmus
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be a bijection
http://en.wikipedia.org/wiki/Bijective_map
Regards,
apfelmus
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:) )
Yes! We'd need such an automatic tool for the wikibook, too.
Regards,
apfelmus
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).
Regards,
apfelmus
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with the formalizer :)
Regards,
apfelmus
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I can perform to make this possible?
Abracadabra, the incantation is
liftIO :: IO a - CGI a
i.e.
parse :: Maybe String- CGI StdGen
parse (Just x) = return $ read x
parse Nothing = liftIO getStdGen
Regards,
apfelmus
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already automated this with Template Haskell or other other
preprocessing tools.
Regards,
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it is not part of the context = [add]
I'm not sure why you'd want to do that, but it's not well-defined. What
would
selectiveQuote [add] ((1 `add` 2) `mul` 3)
be? How to expand `mul` here when `add` isn't expanded?
Regards,
apfelmus
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?
Exactly. Put differently,
writeArray :: STUArray - ..
is strict in the third argument whereas
writeArray :: STArray - ..
is not.
Regards,
apfelmus
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,
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convert :: (a - m b) - m (a - b)
between those two types for a given monad m is equivalent to the
existence of
magic :: ((a - b) - c) - (a - m b) - m c
since we have
convert = magic id
magic f g = return f `ap` convert g
Regards,
apfelmus
...
Oh, and the 8,16,32 and 64 are good candidates for phantom
type/associated data types, too.
Regards,
apfelmus
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mult x x
with
onlyOnce :: IO Int
mult :: Int - Int - IO Int
?
If you want
mult = liftM2 something :: IO Int - IO Int - IO Int
you can
do
x' - onlyOnce
let x = return x'
mult x x
which is
do
x - return `liftM` onlyOnce
mult x x
for short.
Regards,
apfelmus
it via the isomorphism
exists e . expr e = forall w . (forall e . expr e - w) - w
Regards,
apfelmus
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Roberto Zunino wrote:
apfelmus wrote:
cons:: a - List e f a - List Nonempty f a
But unfortunately, finiteness is a special property that the type
system cannot guarantee. The above type signature for cons doesn't
work since the following would type check
bad :: a - List Nonempty
is *hard*, especially since you
can think about it for weeks without touching a computer.
Regards,
apfelmus
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xs)
we can write
fromList :: [a] - ListFinite a
fromList [] = nil
fromList (x:xs) = cons x (fromList xs)
Regards,
apfelmus
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(_,...))) )
and it seems that Hugs fails to evaluate the tail recursive chain of
snd ??
In the end, here's our decisive result: either Hugs or my analysis has a
bug :D
Regards,
apfelmus
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be up to the
programmer's choice (partial reduction may be expensive), so is there a
way to specify that in code?
Regards,
apfelmus
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/gmane.comp.lang.haskell.cafe/26329
Regards,
apfelmus
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at home.
Didn't he say something about the mean formula? Or was it the standard
derivation?).
Regards,
apfelmus
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.
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Tristan Allwood wrote:
Does anyone know if there is a function that tells you if a haskell
value has been forced or not? e.g. isWHNF :: a - IO Bool
apfelmus wrote:
Note that this function [isWHNF :: a - Bool] is not
referentially transparent
Indeed. Does it still mess up with the result
/users_guide/
separate-compilation.html#using-make
for an example Makefile that shows how it's done.
Regards,
apfelmus
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Hello,
the EnableGUI hack for getting a window in GHCi on Mac OS X (10.3.9 for
me) unexpectedly changes the working directory for the running ghci
[...] apfelmus$ ghci -i../../Hackage/SOE/src
[...] GHC Interactive, version 6.6.1, for Haskell 98.
*Main :! pwd
/Users/apfelmus/Documents
Symbol String
Regards,
apfelmus
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how it can be done with Cont
in all cases. It works for the above state monad (*) but what about
primitives like
mplus :: m a - m a - m a
callcc :: ((a - m r) - m r) - m a
that have monadic arguments in a contravariant position (possibly even
universally quantified)?
Regards,
apfelmus
started).
Regards,
apfelmus
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to
Functional Programming using Haskell. For other books, see also
http://haskell.org/haskellwiki/Books_and_tutorials#Textbooks
Regards,
apfelmus
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everything I (thought I) knew (about
programming). The reward was worth it.
Why do people want side effects? Purity is soo much simpler.
Regards,
apfelmus
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no infantilizing.
A stronger condition would be that every valid LearningPrelude program
should be a valid RealHackersPrelude program. This is probably
preferred, but may be tricky due to overloading ambiguities.
Regards,
apfelmus
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Felipe Lessa wrote:
apfelmus wrote:
Of course, the solution is to first drop n elements and then take
tails instead of dropping n elements every time.
map (drop n) . tails = tails . drop n
O(m*n) O(m)
Nice identity. I'll remember this one.
Oops, please don't
,
apfelmus
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the memory reduction for large XML trees which are
likely to have many many identical tag names.
Regards,
apfelmus
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can be translated into
math-speak and are then called parametricity.
Regards,
apfelmus
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- a)
are isomorphic. (And you're right, the only thing this function can do
is to return _|_.)
In contrast, ∃a means I choose a concrete type a at will and you will
have to deal with all of my capricious choices.
Regards,
apfelmus
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about existentials under the terms noted on
http://en.wikibooks.org/wiki/User:Apfelmus. (This also means that I
don't allow to put them on the haskellwiki which has a more liberal
license.)
Thanks for posting this, I finally understand existentials!
λ(^_^)λ
Regards,
apfelmus
Ryan Ingram wrote:
On 10/24/07, apfelmus [EMAIL PROTECTED] wrote:
So, instead of storing a list [∃a. Show a = a], you may as well
store a
list of strings [String].
I've seen this before, and to some extent I agree with it, but it
breaks down for bigger examples due to sharing.
In most cases
)
Regards,
apfelmus
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,
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- ((),*World)
loop w = loop w
loop' w = let (_,w') = print x w in loop' w'
both have denotation _|_ but are clearly different in terms of side
effects. (The example is from SPJs awkward-squad tutorial.) Any pointers?
Regards,
apfelmus
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Brandon S. Allbery KF8NH wrote:
apfelmus wrote:
during function evaluation. Then, we'd need a purity lemma that
states that any function not involving the type *World as in- and
output is indeed pure, which may be a bit tricky to prove in the
presence of higher-order functions
- ... - (*World, ...).
In contrast, we can see IO a as an abstract (co-)data type subject to
some straightforward operational semantics, no need to mess with the
pure - . So, in a sense, the Haskell way is cleaner than the Clean way ;)
Regards,
apfelmus
implement and let some law
intuition guide you. Well-known example: darcs.
Regards,
apfelmus
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rotations like rotL xs n in O(log n) time.
(I keep mixing up the meaning of rotL and rotR , does L push the
current element to the left or does it rotate the ring clockwise?)
Regards,
apfelmus
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to the difficulty of
designing a powerful trading DSL in the first place :)
Regards,
apfelmus
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+ foldr1 (+) [f k | k - [0..n]]
This identity allows us to write
f ∞ = f 1 + foldr1 (+) [f k | k - [0..]]
and hence
rs_bp' = 1: foldr1 (.) mrs' undefined
To close the circle, Alfonso's solution is in fact the deforestation of
this one.
Regards,
apfelmus
$
Context (cycle $ reverse xs) (head xs) (tail $ cycle xs)
Here, mkContexts xs initializes a new infinite cyclic ring for xs
and rotates it left ad infinitum.
Regards,
apfelmus
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(Btw, this ring stuff could be relevant for Xmonad, I don't know whether
the workspace/window-ring implementation there is O(1). Not that it
matters for 1000 windows, of course :)
Justin Bailey wrote:
apfelmus wrote:
Do you really need to realize the cycle by sharing? I mean, sharing
?
The thing you want is called views. See
http://hackage.haskell.org/trac/ghc/wiki/ViewPatterns#Relatedwork
for more.
Regards,
apfelmus
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gmane gradually forget old messages?). The only problem is to
make this work on an infinite list. Dave Bayer discovered a great way to
do this, here's an explanation
http://thread.gmane.org/gmane.comp.lang.haskell.cafe/26426/focus=26493
Regards,
apfelmus
Brent Yorgey wrote:
apfelmus, does someone pay you to write so many thorough, insightful and
well-explained analyses on haskell-cafe? I'm guessing the answer is 'no',
but clearly someone should! =)
Depending on length, my prices for posts range between λ9.99 and λ29.99 ;)
Regards,
apfelmus
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