Omitting the typeclass bit, I'm trying to write something like
(s1 -> s2) -> StateT s1 m () -> StateT s2 m a -> StateT s1 m a
That is, it sequences two StateT computations, providing a way to
translate from the first's state to the second to keep the chain
going.
I can easily write something for
Hi, I've got a few (9) random questions, mainly about monads and building
monads from existing monads, partly trying to confirm conclusions I've come
to through experimentation.
Any, and all, attempts to enlighten me will be much appreciated.
Thanks
Daniel
First, terminology. I
On 23/03/06, Daniel McAllansmith <[EMAIL PROTECTED]> wrote:
> Hi, I've got a few (9) random questions, mainly about monads and building
> monads from existing monads, partly trying to confirm conclusions I've come
> to through experimentation.
>
> Any, and all, a
Oh, and almost forgot, you can check out lots of examples of this not
only in the mtl, but also on the (old) Haskell Wiki. I've written a
lot of simple (sometimes trivial) examples for people to look at
Unique values -- very simple
http://www.haskell.org/hawiki/MonadUnique
A supply of values, a s
On Friday 24 March 2006 16:42, Cale Gibbard wrote:
Excellent help thanks, Cale.
A lot of my misunderstandings stemmed from not finding any 'instance
MonadState ReaderT' when reading the code in Reader.hs, not realising that
there was an instance defined in State.hs, and yet being able to use ge
On Friday 24 March 2006 16:49, Cale Gibbard wrote:
> Oh, and almost forgot, you can check out lots of examples of this not
> only in the mtl, but also on the (old) Haskell Wiki. I've written a
> lot of simple (sometimes trivial) examples for people to look at
I'll be sure to check them out.
> La
What are Monads?
Where can I read about then?
I read that there´s some mathematical foundation under this concept, where
can read about these theories?
+-+
| Marcos Aurélio Almeida da Silva |
| E-mail: [EMAIL PROTECTED
Not really seeing why Unique is in the IO monad, not deeply understanding
the use of Haskell extensions in the State source, and wanting to try to
learn a bit more about monads, I thought I'd try to write my own monad for
the first time: something for producing a series of unique labels. Th
Mark,
I'm no expert, but does it help to start from withStateT?
> withStateT :: (s -> s) -> StateT s m a -> StateT s m a
> withStateT f m = StateT $ runStateT m . f
There are some notes about computations and lifting
state transformers in
Modular Denotational Semantics for Compiler Construction
Mark Carroll <[EMAIL PROTECTED]> wrote in article <[EMAIL PROTECTED]> in
gmane.comp.lang.haskell.cafe:
> Omitting the typeclass bit, I'm trying to write something like
> (s1 -> s2) -> StateT s1 m () -> StateT s2 m a -> StateT s1 m a
>
> That is, it sequences two StateT computations, providing a w
On Wed, 31 Dec 2003, Ken Shan wrote:
> Don't you need a (s2 -> s1) function as well, to translate the final
> state back into StateT s1?
Yes, you're right: the thing actually running the stateful computation
presumably expects to start it with a state of type s1 and to be able to
extract from it
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Let me offer a differing view of Monads.
Monads are a way to write type-safe imperative programs within a functional
framework. It's just an advanced version of PROGN kludge in LISP.
Since they are based on a linear flow of "commands&q
"Eray Ozkural (exa)" wrote:
>
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> Hash: SHA1
>
> Let me offer a differing view of Monads.
>
> Monads are a way to write type-safe imperative programs within a functional
> framework. It's just an advanced version of PR
At 12:47 PM -0600 1/16/02, Eray Ozkural (exa) wrote:
>
>Let me offer a differing view of Monads.
>
>Monads are a way to write type-safe imperative programs within a functional
>framework. It's just an advanced version of PROGN kludge in LISP.
>
>Since they are based on
Artie Gold writes:
>One way to think of it is to look at a program as a partially ordered
>set of calculations; some calculations need to occur before others,
>other groups can occur in any order. In an imperative language you
>specify a total ordering (which is overkill).
This is a weak argumen
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Hi Richard,
On Tuesday 19 February 2002 06:57, Richard Uhtenwoldt wrote:
>
> This is a weak argument.
>
> First of all it is not the case that imperative coders always specify a
> total ordering: multitasking, threading and interrupts (and their
> pr
Hi
Does the list consider
http://en.wikibooks.org/w/index.php?title=Haskell/Understanding_monads&oldid=933545
a reliable tutorial on monads and, if not, could you recommend an
onlien alternative please?
Thanks,
Paul
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I see a lot of literature that says that monads "simulate" the effects of
imperative programming concepts. It seems to me that the relative
performance of monadic implementations must be equivalant to imperative
ones to provide a strong case for functional programming. For examp
In a message dated 12/12/2002 2:45:31 PM Eastern Standard Time, [EMAIL PROTECTED]
writes:
> What are Monads?
> Where can I read about then?
>
> I read that there´s some mathematical foundation under this
> concept, where
> can read about these theories?
A large set of art
I've found this page a very helpful introduction to Monads and monadic
programming.
http://www.dcs.gla.ac.uk/~nww/Monad.html
Have fun,
Joel.
On Thu, 2002-12-12 at 19:45, [EMAIL PROTECTED] wrote:
> What are Monads?
> Where can I read about then?
>
> I read that theres
Hi Joel and Jim - thanks for your useful responses to Marcos' question.
I've added them to the UsingMonads page of the Haskell Wiki,
http://www.haskell.org/wiki/wiki?UsingMonads
Hope that's OK. Feel free to edit or extend or add your names if you
like.
To everyone else on the list: please ad
OTECTED]
>
> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of Haskell-Cafe digest..."
>
>
> Today's Topics:
>
>1. Combining monads (Mark T.B. Carroll)
>2. Re: Combining monads (Scott Turner)
>3. Re: Combini
On Saturday 12 April 2003 23:52, Dominic Steinitz wrote:
> For the theory, look at
> Triples, Toposes and Theories by Barr & Wells. I think it's chapter 7 but I
> haven't got it in front of me.
>
This looks like it's quickly getting out of the realm of programming :)
Cheers,
--
Eray Ozkural (ex
> (a) People could point out to me where I'm still confused, as
> revealed by
> my code. Is it needlessly complicated?
looks pretty reasonable to me :)
as to why Unique is in the IO monad is probabyl because if it were in
any other monad, you could start the monad twice and thus get a repeat
of
e
necessary because the state type depends on the monad (get would have
type forall s.Monad m => m -> s otherwise which is rather meaningless),
the function dependencies tell the type checker that the state type is
completely determined by the monad type.
> and
> wanting to try to
not deeply
understanding the use of Haskell extensions in the State source,
I'm assuming Control.Monad.State's source in which case -no- extensions
are used for -State- (well, at least I don't see any quickly glancing).
Extensions are used for the -MonadState class-.
The portable parts of Con
The previous "newtype Labeller a = Labeller (Int -> (Int, a))" (the
result tuple is reversed within Control.Monad.State) would simply become
(untested):
newtype Labeller a = State Int a
newLabel = do { n <- get; put (n + 1); return (Label n) }
runLabeller l = execState l minBound
it must be "e
On Friday, 2003-06-27, 12:55, CEST, Christian Maeder wrote:
> [...]
> The portable parts of Control.Monad.State (that are sufficient for most
> cases) should be in an extra module (maybe called Control.Monad.StateTypes).
> In addition further non-overloaded names for put, get, gets and modify woul
On Thursday, 2003-06-26, 23:57, CEST, Derek Elkins wrote:
> [...]
> > not deeply understanding the use of Haskell extensions in the State
> > source,
>
> I'm assuming Control.Monad.State's source in which case -no- extensions are
> used for -State- (well, at least I don't see any quickly glancing)
rstanding
the use of Haskell extensions in the State source, and wanting to try to
learn a bit more about monads, I thought I'd try to write my own monad for
the first time: something for producing a series of unique labels. This is
how it turned out:
=
The portable parts of Control.Monad.State (that are sufficient for most
cases) should be in an extra module (maybe called Control.Monad.StateTypes).
In addition further non-overloaded names for put, get, gets and modify would
be needed (maybe putState, getState, etc.)
I fear, this would complicate
I have been following the recent "Monad tutorial" discussion with interest,
and even read the tutorial, which is a useful addition to the existing
Haskell documentation. So useful in fact that it raises a question...
The whole monad mechanism seems to geared towards functions of one argument,
p
Hello,
I would an examples of monads that are pure, i.e. no side-effects.
Thank you,
Vasili
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Hi!
Suppose I want to create a specific monad as a combination of monad transformers - something like "StateT smth1 (ReaderT smth2 Identity)". As
you can see, each transformer is parametrized with a type of kind *. I want to abstract these parameters, so that instead of "StateT smth..." I
can w
I've been getting some pretty weird complaints from the type checker.
And I just figured out why! Grr...
Is there any function that does the same thing as "until", but in a monad?
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Take a look at the following:
data Writer o v = Writer [o] v
instance Monad (Writer o) where
return v = Writer [] v
(Writer os v) >>= f =
let (Writer os' v') = f v
in Writer (os ++ os') v'
write :: o -> Writer o ()
write o = Writer [o] ()
writes :: [o] -> Writer o ()
wri
HaskellyCaffeinated,
i noticed that there was a JavaMonad lib kicking around on the web, but all
the links i can find are stale. Does anybody have a live pointer to this
lib?
Best wishes,
--greg
--
L.G. Meredith
Managing Partner
Biosimilarity LLC
505 N 72nd St
Seattle, WA 98103
+1 206.650.374
7; is better than 'IO a->b'.
The full title of this is really 'Explaining monads by comparison
with comonads and arrows', but I didn't want to scare people off
without putting in the above hook!
We start with a simple single value of type 'a', and then we
ntially that function in every real program? - Greg
- Original Message
From: Jeff Polakow <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Cc: [EMAIL PROTECTED]; Haskell-Cafe
Sent: Tuesday, August 14, 2007 8:45:06
AM
Subject: Re: [Haskell-cafe] Explaining monads
One general intu
Haskellians,
Is there a characterization of "prime" monads? Here the notion of
factorization i'm thinking about is decomposition into adjoint situations.
For example, are there monads for which there are only the Kleisli and
Eilenberg-Moore decompositions into adjoint situations?
Hello All,
Being at the "Hello Monads" level, and having spent many hours on
understanding the following code from YAHT, maybe this will be helpful
to others.
{-cross ls1 ls2 = do
x <- ls1
y <- ls2
return (x,y)
-}
In the List Mo
another beginners question about monads: given the type
| data (Ix x) => Permutation x = Permutation [x]
i wanted to define
| instance Monad Permutation where
| return xs = Permutation xs
but of course nothing about the monad class guarantees xs to be of
type list. the monad class se
Hi All,
Next monad query [*]
In the 1995 paper "Composing Haggis", layout is done using a monad to
compose individual elements. To modernize the syntax consider (forgive
the operator, but it avoids parentheses):
infixl 1 <|
f <| x = f x
mylayout
= do
hbox <| do
button
Matthias Fischmann wrote:
> another beginners question about monads: given the type
>
> | data (Ix x) => Permutation x = Permutation [x]
>
> i wanted to define
>
> | instance Monad Permutation where
> | return xs = Permutation xs
>
> but of course nothing about
Matthias Fischmann wrote:
> > Do you expect the contained type x to change during a
> > sequence of monadic actions? e.g. would you ever use
(>>=)
> > at the type 'Permutation Int -> (Int -> Permutation
Bool) ->
> > Permutation Bool'?
>
> no, i don't need that. but aside from
> the fact that
>
>
I was interested to read David Espinosa's "Stratified Monads" paper at
http://www-swiss.ai.mit.edu/~dae/papers/sm.ps.Z
I'm not sure I actually understand them properly yet, but I'm already
curious about if anybody's played with them in Haskell, or how useful
it w
rking
correctly... I'd really prefer to just layer error handling on the top.
But I just can't get it to work right. It's s fiddly untangling the
multiple monads to try to *do* any useful work.
Does anybody have any idea how this whole monad stacking craziness is
*supposed* to work?
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Hi,
If I have two computations a->IO b
and b->IO c, can I join them to
get an a->IO c computation? I imagine
something like a liftM dot operator.
Thanks,
Maurício
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Hi,
2007/12/3, PR Stanley <[EMAIL PROTECTED]>:
> Hi
> Does the list consider
> http://en.wikibooks.org/w/index.php?title=Haskell/Understanding_monads&oldid=933545
> a reliable tutorial on monads and, if not, could you recommend an
> onlien alternative please?
I really
PR Stanley wrote:
> Does the list consider
>
http://en.wikibooks.org/w/index.php?title=Haskell/Understanding_monads&oldid=933545
> a reliable tutorial on monads and, if not, could you recommend an
> onlien alternative please?
I strongly recommend the original papers by Philip Wa
Constructing some code today in Python, using some functional-style coding
idioms, I found myself wondering if there would be any real benefit to using a
monad-based implementation (i.e. other than to demonstrate that it can be done).
The application that sparked this line of thought was a simple
Monads in Haskell are a topic that I, like most beginners, find
difficult and "mind-twisting". Now that I think I understand monads,
they seem to be very simple; I've read that this is a common
experience.
So I wonder, what would it take to help beginners catch on with a
min
I have a solution so this is for interest only.
It is not normally the case that two monads compose to give another
monad. Monad transformers capture when this is possible. However, when
there is a "swap" function satisfying some commutative diagrams then it
can be proved that the mona
Hello!
Can anybody help me with writing function that does
following:
I have the list ["file1","file2","file3"] and I need the
function to return me the list of contents of these files.
contents :: [String] -> [String]
contents (file:files) = do c <- readFile file
Thanks in advance
Pixel <[EMAIL PROTECTED]> asks:
> I'm trying to achieve something like (ocaml)
> [code deleted]
> which is a foldl on 2 iterators. I've managed to use monads for one iterator,
> but i completly miss the way to work with 2 monads. Is there really no other
> solu
I'm trying to achieve something like (ocaml)
let rec zipfold f init it1 it2 =
if it1#is_end || it2#is_end then init
else
let v1, v2 = it1#value, it2#value in
it1#next ; it2#next ;
zipfold f (f v1 v2 init) it1 it2
which is a foldl on 2 iterators. I've managed to use
Eric Allen Wohlstadter <[EMAIL PROTECTED]> wrote,
> I see a lot of literature that says that monads "simulate" the effects of
> imperative programming concepts. It seems to me that the relative
> performance of monadic implementations must be equivalant to imperative
&
2002 at 08:59:14PM +1100, Manuel M. T. Chakravarty wrote:
> PS: These optimisations will usually only apply to monads
> that are defined as part of the system libraries of a
> Haskell system, not to user-defined ones (unless a user
> uses non-standard system features to impleme
> I see a lot of literature that says that monads "simulate" the effects of
> imperative programming concepts.
I think that's just bad wording. To take a rather trite
point of view, in a language such as C /everything/ is done
within a monad, and all types, even int, are re
Eric Allen Wohlstadter wrote:
> I see a lot of literature that says that monads "simulate" the effects of
> imperative programming concepts. It seems to me that the relative
> performance of monadic implementations must be equivalant to imperative
> ones to provide a stro
> I agree with others who mentioned that viewing monads as simply
> providing a way to sequentialize things or to program imperatively is
> the wrong way to look at them.
Yes, Lists are the classical example.
> That said, the EFFICIENCY of monads is often poorly understood. To
I've felt need to use State monads in two distinct situations:
...
And I've seen two distict aproaches. Using a State monad like one
provided with GHC (1), or a state monad like the one defined in the
paper "Moands for the Working Haskell
On Thursday 24 January 2002 14:09, John Hughes wrote:
> I've felt need to use State monads in two distinct situations:
> ...
>
> And I've seen two distict aproaches. Using a State monad like one
> provided with GHC (1), or a state monad like the one def
this will be my last message on this topic as I need to stop
reading this list for a few months.
Alastair Reid writes:
>A potential difference (which Richard Uhtenwoldt hints at) is that it
>can be hard to control the flow of OS capabilities as the capability
>is passed from one process to anoth
On Tuesday, 2003-08-19, 12:42, CEST, Konrad Hinsen wrote:
> I have been following the recent "Monad tutorial" discussion with interest,
> and even read the tutorial, which is a useful addition to the existing
> Haskell documentation. So useful in fact that it raises a question...
>
> The whole mona
Hi
> > As an example, I'll use the Maybe monad. Suppose I want to write code to
> > handle experimental data, in which there might be missing values. I might
> > then decide to represent measurements by data of type "Maybe Double", with
> > missing values represented by "Nothing". I could then go
On Tue, 19 Aug 2003 14:09:16 +0100 (BST)
C T McBride <[EMAIL PROTECTED]> wrote:
> Hi
>
> > > As an example, I'll use the Maybe monad. Suppose I want to write
> > > code to handle experimental data, in which there might be missing
> > > values. I might then decide to represent measurements by data
h) => Fun (Comp g h) where
eta x = Comp (eta (eta x))
Comp ghf <$> Comp ghs = Comp (eta (<$>) ghf <$> ghs)
That's to say, you can define <$> for the composition of two Funs, hence
of two Monads, but, if I recall correctly, it's rather harder to define
>
uot;. Is there any mechanism to handle
> that?
Yes. Many complicated proposals have been made, but there's a
straightforward, general mechanism:
> addMaybe :: Num alpha => Maybe alpha -> Maybe alpha -> Maybe alpha
> addMaybe a b = a >>= \x ->
>b >&g
tor 2003-08-21 klockan 22.26 skrev Jon Cast:
> Yes. Many complicated proposals have been made, but there's a
> straightforward, general mechanism:
>
> > addMaybe :: Num alpha => Maybe alpha -> Maybe alpha -> Maybe alpha
> > addMaybe a b = a >>= \x ->
> >b >>= \y ->
> >
On dinsdag, aug 19, 2003, at 15:09 Europe/Amsterdam, C T McBride wrote:
Hi
As an example, I'll use the Maybe monad. Suppose I want to write
code to
handle experimental data, in which there might be missing values. I
might
then decide to represent measurements by data of type "Maybe
Double", wi
> tor 2003-08-21 klockan 22.26 skrev Jon Cast:
> > Yes. Many complicated proposals have been made, but there's a
> > straightforward, general mechanism:
> >
> > > addMaybe :: Num alpha => Maybe alpha -> Maybe alpha -> Maybe alpha
> > > addMaybe a b = a >>= \x ->
> > >b >>= \y ->
>
turn f) => Monad f where
(>>=) :: f a -> (a -> f b) -> f b
fail :: String -> f a;
fail = error;
Certain functions that seem to require Monads actually work with any
FunctorApplyReturn. For instance:
class (Functor f) => ExtractableFunctor f where
fExtract :: (F
On Thu, Aug 21, 2003 at 11:32:47AM +0100, C T McBride wrote:
> My point, however, is not to use <$> with that type, but the more general
>
> class Fun f where
> eta :: x -> f x
> (<$>) :: f (s -> t) -> f s -> f t
>
> Is there a better name for Fun? Is it ancient and venerable? Am I an
>
g (Show,Eq)
> instance Monoidal s => Fun (K s) where
> eta _ = K m0
> K x <$> K y = K (x <+> y)
> infixl 9
> () :: (Functorial g,Monoidal s) => (x -> s) -> g x -> s
> f gx = unK ((K . f) <^> gx)
*** 3 what extra syntax might be nic
I have an extremely-newbie question about monads and how to interpret
the monadic laws; I asked that same question yesterday on IRC and the
answers were interesting but non-conclusive (to me anyway).
I'm trying to learn monads by reading "All About Monads", version 1.0.2.
I thoug
You have fallen into the misconception that monads are impure, they are not.
Many monad tutorials begin (erroneously) with the lines "monads allow
you to do impure programming in Haskell."
This is false, monads are pure, it's IO that's impure, not the monadic
programming st
From:
http://www.haskell.org/haskellwiki/Monad
"The computation doesn't have to be impure and can be pure itself as
well. Then monads serve to provide the benefits of separation of
concerns, and automatic creation of a computational "pipeline"."
On Sat, Sep 29,
Vasili I. Galchin wrote:
I would an examples of monads that are pure, i.e. no side-effects.
One view of programming in monadic style is: You call return and >>= all
the time. (Either you call it directly, or do notation calls it for
you). So if you want to understand whether a mona
On 12-09-29 09:57 PM, Vasili I. Galchin wrote:
I would an examples of monads that are pure, i.e. no side-effects.
What does "side effect" mean, to you? Definition?
Because some people say "State has no side effect", and some other
people say "State has side
On 9/30/12 7:00 AM, Tillmann Rendel wrote:
Vasili I. Galchin wrote:
I would an examples of monads that are pure, i.e. no side-effects.
One view of programming in monadic style is: You call return and >>= all
the time. (Either you call it directly, or do notation calls it for
you). So
On Sep 30, 2012 10:56 AM, "Albert Y. C. Lai" wrote:
>
> On 12-09-29 09:57 PM, Vasili I. Galchin wrote:
>>
>> I would an examples of monads that are pure, i.e. no
side-effects.
>
>
> What does "side effect" mean, to you? Definition?
When d
On Sun, Sep 30, 2012 at 6:33 PM, Jake McArthur wrote:
>
> On Sep 30, 2012 10:56 AM, "Albert Y. C. Lai" wrote:
>>
>> On 12-09-29 09:57 PM, Vasili I. Galchin wrote:
>>>
>>> I would an examples of monads that are pure, i.e. no
>>> side
On 12-09-30 06:33 PM, Jake McArthur wrote:
When discussing monads, at least, a side effect is an effect that is
triggered by merely evaluating an expression. A monad is an interface
that decouples effects from evaluation.
I don't understand that definition. Or maybe I do subconsciousl
"Albert Y. C. Lai" writes:
> On 12-09-30 06:33 PM, Jake McArthur wrote:
>> When discussing monads, at least, a side effect is an effect that is
>> triggered by merely evaluating an expression. A monad is an interface
>> that decouples effects from evaluati
On 12-10-01 05:34 AM, Jon Fairbairn wrote:
"Albert Y. C. Lai" writes:
On 12-09-30 06:33 PM, Jake McArthur wrote:
When discussing monads, at least, a side effect is an effect that is
triggered by merely evaluating an expression. A monad is an interface
that decouples effects from
mputation would be bound to
operate on the same type, would this be without any undesirable
implications?
For the sake of understanding monads better, I tried to write several
custom monads which may or may not be useful. Among those were:
* The Tracker Monad - tracks every result of every s
- join being a part of the fundamental theoretical
structure of monads:
join :: Monad m => m (m a) -> m a
For the sake of understanding monads better, I tried to write several
custom monads which may or may not be useful. Among those were:
* The Tracker Monad - tracks every result of
Geoffrey Alan Washburn <[EMAIL PROTECTED]> writes:
> Scala can do much better still because it has first-class functions and
> algebraic data types ("case classes").
Comments on http://lambda-the-ultimate.org/node/view/1136 include links to
Scala http://scala.epfl.ch/examples/files/simpleInterpr
Q: Are the "continuations" in Scheme related to the "monads" from
Haskell? If so, could someone elaborate on that?
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I recognized that Janis Voigtlaender defines the type `ContT`
under the name `C` in Section 3 of his paper on "Asymptotic
Improvement of Computations over Free Monads" (available at
http://wwwtcs.inf.tu-dresden.de/~voigt/mpc08.pdf) and gives a monad
instance without constraints on the f
Miguel Mitrofanov wrote:
Suppose I want to create a specific monad as a combination of monad
transformers - something like "StateT smth1 (ReaderT smth2 Identity)".
As you can see, each transformer is parametrized with a type of kind *.
I want to abstract these parameters, so that instead of "St
Probably.
I have two objections against using type families. Both are pretty
much theoretical.
First, it seems to me that using type families would require some
other extensions. Multi-parameter type classes are OK, but, in my
experience, the road from them to the darkness of undecidable
Miguel Mitrofanov wrote:
First, it seems to me that using type families would require some other
extensions. Multi-parameter type classes are OK, but, in my experience,
the road from them to the darkness of undecidable instances is quite
short, and I don't feel very safe on these grounds.
Act
What do you mean, without instances? How do you call "data instance"
declarations?
On 16 Jun 2009, at 02:16, Ashley Yakeley wrote:
Miguel Mitrofanov wrote:
First, it seems to me that using type families would require some
other extensions. Multi-parameter type classes are OK, but, in my
e
at, 19 May 2007 19:49:40 +0100
Subject: [Haskell-cafe] Pesky monads...
I've been getting some pretty weird complaints from the type checker.
And I just figured out why! Grr...
Is there any function that does the same thing as "until", but in a monad?
__
exit the loop instead... Oh,
wait, for some monads that fires an error. Hmm. Nice try.
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else f x >>= untilM pred f
Think of a computation in the State monad, a predicate will probably want to
consult that state.
Matthew Cox
- Original Message -
From: Matthew Cox
To:
Sent: Sat, 19 May 2007 13:10:40 -0600
Subject: Re: [Haskell-cafe] Pesky monads...
You can define one:
I previously worked out how to use the monad transformers to make a when /
repeat control structure that admitted both break and continue
statements. It uses a ContT monad transformer to provide the escape semantics
and the Reader to store the continuation.
I'll paste the code here:
> -- By Chri
matt:
> It occurred to me that the predicate will generally be a monadic function
> itself, so here's a
> refined version:
>
> :: Monad m => (a -> m Bool) -> (a -> m a) -> a -> m a
> untilM pred f x = do c <- pred x
> if c then return x
> else f x >
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