Thanks for pointing that out.<br><br>I haven't been seriously dealing with the monad laws as the code is not even working yet.<br>But I guess it should be able to adjust the definition to fit the monad laws.<br><br>Say, with current definition we have<br>return 1 >>= return<br>===<br>WH ([],1) >>= \x->WH ([],x)<br>===<br>WH ([1],1)<br>But if we change the definition of >>= to<br>WH (h,a) >>= fm = WH (h1++if a == b then coerceHistory h else coerceHistory (a:h),b) where ...<br>then <br>return 1 >>= return<br>===<br>WH ([],1)<br>while<br>return 1 >>= return.(+2)<br>===<br>WH ([1],3)<br>Of course this may break in other cases.<br><br>The reason is that (apart from reducing explicit recording) I have used the Maybe monad extensively <br>to express "computation that may fail" in existing code, and in the end I would like something which not only <br>handles the possible failure also records history based on this WithHistory monad.<br><br>在 2018-11-19 01:34:33，"Dan Burton" <danburton.email@gmail.com> 写道：<br> <blockquote id="isReplyContent" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid"><div dir="ltr"><div dir="ltr"><div dir="ltr">I would recommend against this, since having >>= "record history" breaks monad laws. Particularly, this one:<br><br>f >>= return === f<div><br>Instead, why not use a plain old `Writer [Dynamic]` monad and explicitly `tell` whenever you want to add a historical record?<br><br><div><div dir="ltr" class="gmail_signature"><div dir="ltr" class="gmail_signature"><font face="monospace, monospace">import Control.Monad.Trans.Writer (Writer, tell)</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace">import Data.Dynamic (Dynamic, toDyn)</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace">import Data.Typeable (Typeable)</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"><br></font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace">newtype WithHistory b = WH (Writer [Dynamic] b)</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"> deriving (Functor, Applicative, Monad)</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"><br></font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace">tellHistory :: Typeable a => a -> WithHistory ()</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace">tellHistory a = WH $ tell [toDyn a]</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"><br></font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace">someComputation :: WithHistory Int</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace">someComputation = do</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"> let x = 1</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"> tellHistory x</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"> let y = x + 1</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"> tellHistory y</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"> let yStr = show y</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"> tellHistory yStr</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"> let z = y + 3</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"> tellHistory z</font></div><div dir="ltr" class="gmail_signature"><font face="monospace, monospace"> return z</font></div><div><br></div><br>-- Dan Burton</div></div><br><br><input name="virtru-metadata" type="hidden" value="{"email-policy":{"state":"closed","expirationUnit":"days","disableCopyPaste":false,"disablePrint":false,"disableForwarding":false,"enableNoauth":false,"expires":false,"isManaged":false},"attachments":{},"compose-id":"3","compose-window":{"secure":false}}"></div></div></div><br><div class="gmail_quote" style=""><div dir="ltr">On Sun, Nov 18, 2018 at 10:31 AM ducis <<a href="mailto:ducis_cn@126.com">ducis_cn@126.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="line-height:1.7;color:#000000;font-size:14px;font-family:Arial"><div>Hi, Anthony,<br><br>The top-level story is that I am trying to create a monad that somehow records the "intermediate steps" of computation.<br>e.g. something like<br>Prelude> return 1 <br></div><div>([],1)<br>Prelude> return 1 >>= return.(+1)<br>([1],2)<br>Prelude> return 1 >>= return.(+1)>>=return.(+3)<br>([2,1],5)<br></div><div>(the list has the intermediate steps placed right-to-left so that new steps are appended to the left of the older steps)<br>Of course all "intermediate steps of computation" actually form a graph, but we are frequently focused on, say, <br>the transformation of a parse tree, where we want to take a series of snapshots of one "thing".<br><br>Since a "lifted function" (e.g. return.(+1)) has in general the type a->m b, there are two ways<br>to deal with input and output being not necessarily equal.<br><br>The first approach I tried is to only record latest steps starting with the last change of type<br>> newtype WithHistory b = WH ([b], b) <br></div><div>and just discard the older steps when the input and output are of different types.<br>> newtype WithHistory b = WH ([b], b) deriving (Show,Eq) <br>> instance Monad WithHistory where <br>> return b = WH ([], b) <br>> (>>=) :: forall a b. WithHistory a -> (a -> WithHistory b) -> WithHistory b <br>> WH (h,a) >>= fm = WH (h1++coerceHistory (a:h),b) <br>> where <br>> WH (h1, b) = fm a <br>> class CoerceHistory a b where <br>> coerceHistory :: [a] -> [b] <br>> instance CoerceHistory a a where <br>> coerceHistory = id <br>> instance CoerceHistory a b where <br>> coerceHistory _ = []<br>I have got the coerceHistory function to (appear to) work in GHCi<br>*Main> coerceHistory [2::Int] :: [Int] <br>[2] <br>*Main> coerceHistory "c" :: [Int] <br>[] <br>But the Monad instanciation does not really work.<br>GHC(7.6.3) hints for -XIncoherentInstances, which when<br>enabled seems to force the (>>=) to always use the instance<br>of coerceHistory returning []<br><br>The second approach is to use [Dynamic] for steps, i.e.,<br>> newtype WithHistory b = WH ([Dynamic], b)<br>> instance Monad WithHistory where<br>> return b = WH ([], b) <br>> WH (h,a) >>= fm = WH (h1++forceDynList a++h, b)<br>> where WH (h1, b) = fm a<br>and presumably<br></div><div>> class ForceDynList a where forceDynList :: a -> [Dynamic] <br>> instance (Typeable a) => ForceDynList a where forceDynList x = [toDyn x]<br>> instance ForceDynList a where forceDynList x = []<br>which is far from correct with error "Duplicate instance declarations"<br><br>Thanks! <br>Ducis<br></div><br><br><div style="zoom:1">--<br>-----------------------------<br><div style="clear:both"></div></div><div id="m_-4385674626126640850divNeteaseMailCard"></div><br><pre><br>At 2018-11-18 20:00:01, <a href="mailto:haskell-cafe-request@haskell.org" target="_blank">haskell-cafe-request@haskell.org</a> wrote:
>Send Haskell-Cafe mailing list submissions to
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>Today's Topics:
>
> 1. Timing out a pure evaluation of an expression I did not write
> myself (Ryan Reich)
> 2. Re: Timing out a pure evaluation of an expression I did not
> write myself (Daniel Díaz Casanueva)
> 3. Re: Timing out a pure evaluation of an expression I did not
> write myself (Daniel Díaz Casanueva)
> 4. Re: Timing out a pure evaluation of an expression I did not
> write myself (Ryan Reich)
> 5. Specialize a function on types of arguments? (ducis)
> 6. Re: Specialize a function on types of arguments? (Anthony Clayden)
> 7. Re: Timing out a pure evaluation of an expression I did not
> write myself (<a href="mailto:arjenvanweelden@gmail.com" target="_blank">arjenvanweelden@gmail.com</a>)
> 8. external git dependency source in .cabal (Fabien R)
>
>
>----------------------------------------------------------------------
>
>Message: 1
>Date: Sat, 17 Nov 2018 15:21:53 -0800
>From: Ryan Reich <<a href="mailto:ryan.reich@gmail.com" target="_blank">ryan.reich@gmail.com</a>>
>To: haskell-cafe <<a href="mailto:haskell-cafe@haskell.org" target="_blank">haskell-cafe@haskell.org</a>>
>Subject: [Haskell-cafe] Timing out a pure evaluation of an expression
> I did not write myself
>Message-ID:
> <CAO8Ocku+=6GkyW-z8bShr7OSJppT_yPmrkcknF=<a href="mailto:UXYDn7%2BKBQg@mail.gmail.com" target="_blank">UXYDn7+KBQg@mail.gmail.com</a>>
>Content-Type: text/plain; charset="utf-8"
>
>I want to time out a pure computation. My experience, and that described
>in various previous questions here and elsewhere (the best of which is
><a href="https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html" target="_blank">https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html</a>),
>is that this doesn't always work: for instance,
>
>>>> timeout 1 $ evaluate $ let x = 0 : x in last x
>
>does not time out because, apparently, the fact that the expression
>evaluates in constant space (i.e. never allocates) means that it never
>yields to the timeout monitor thread that would kill it.
>
>The solution that is described in the other iterations is to embed
>checkpoints in the expression that do allocate, giving the RTS a chance to
>switch contexts. However, in my application, the expression is /arbitrary/
>and I do not have the freedom to inject alterations into it. (Don't argue
>this point, please. The expression is arbitrary.)
>
>How can I time out a tight loop like the above? Clearly, it can be done,
>because I can, say, alt-tab over to another terminal and kill the process,
>which exploits the operating system's more aggressively pre-emptive
>scheduling. Is there a solution using bound threads, say 'forkOS' instead
>of 'forkIO' in the implementation of 'timeout'? Unix signals? Some
>FFI-based workaround? Etc. Keep in mind that notwithstanding that
>comment, I don't actually want to kill the whole process, but just the one
>evaluation.
>
>Thanks in advance,
>Ryan Reich
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>------------------------------
>
>Message: 2
>Date: Sun, 18 Nov 2018 01:51:58 +0100
>From: Daniel Díaz Casanueva <<a href="mailto:dhelta.diaz@gmail.com" target="_blank">dhelta.diaz@gmail.com</a>>
>To: <a href="mailto:ryan.reich@gmail.com" target="_blank">ryan.reich@gmail.com</a>
>Cc: haskell-cafe <<a href="mailto:haskell-cafe@haskell.org" target="_blank">haskell-cafe@haskell.org</a>>
>Subject: Re: [Haskell-cafe] Timing out a pure evaluation of an
> expression I did not write myself
>Message-ID:
> <<a href="mailto:CAJFXAQimv-mJB7e1WMzz1urduRr5MV5mewVhhaCH52rFxJtY1w@mail.gmail.com" target="_blank">CAJFXAQimv-mJB7e1WMzz1urduRr5MV5mewVhhaCH52rFxJtY1w@mail.gmail.com</a>>
>Content-Type: text/plain; charset="utf-8"
>
>Hello Ryan.
>
>Try evaluating the expression to normal form instead of weak head normal
>form in your expression. So:
>
>>>> timeout 1 $ evaluate $ force $ let x = 0 : x in last x
>
>The function `force` comes from the deepseq package. You can read the docs
>here:
><a href="http://hackage.haskell.org/package/deepseq-1.4.4.0/docs/Control-DeepSeq.html" target="_blank">http://hackage.haskell.org/package/deepseq-1.4.4.0/docs/Control-DeepSeq.html</a>
>
>I hope that helps.
>
>Best regards,
>Daniel
>
>Am So., 18. Nov. 2018 um 00:22 Uhr schrieb Ryan Reich <<a href="mailto:ryan.reich@gmail.com" target="_blank">ryan.reich@gmail.com</a>
>>:
>
>> I want to time out a pure computation. My experience, and that described
>> in various previous questions here and elsewhere (the best of which is
>> <a href="https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html" target="_blank">https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html</a>),
>> is that this doesn't always work: for instance,
>>
>> >>> timeout 1 $ evaluate $ let x = 0 : x in last x
>>
>> does not time out because, apparently, the fact that the expression
>> evaluates in constant space (i.e. never allocates) means that it never
>> yields to the timeout monitor thread that would kill it.
>>
>> The solution that is described in the other iterations is to embed
>> checkpoints in the expression that do allocate, giving the RTS a chance to
>> switch contexts. However, in my application, the expression is /arbitrary/
>> and I do not have the freedom to inject alterations into it. (Don't argue
>> this point, please. The expression is arbitrary.)
>>
>> How can I time out a tight loop like the above? Clearly, it can be done,
>> because I can, say, alt-tab over to another terminal and kill the process,
>> which exploits the operating system's more aggressively pre-emptive
>> scheduling. Is there a solution using bound threads, say 'forkOS' instead
>> of 'forkIO' in the implementation of 'timeout'? Unix signals? Some
>> FFI-based workaround? Etc. Keep in mind that notwithstanding that
>> comment, I don't actually want to kill the whole process, but just the one
>> evaluation.
>>
>> Thanks in advance,
>> Ryan Reich
>> _______________________________________________
>> Haskell-Cafe mailing list
>> To (un)subscribe, modify options or view archives go to:
>> <a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe</a>
>> Only members subscribed via the mailman list are allowed to post.
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>
>------------------------------
>
>Message: 3
>Date: Sun, 18 Nov 2018 01:55:56 +0100
>From: Daniel Díaz Casanueva <<a href="mailto:dhelta.diaz@gmail.com" target="_blank">dhelta.diaz@gmail.com</a>>
>To: <a href="mailto:ryan.reich@gmail.com" target="_blank">ryan.reich@gmail.com</a>
>Cc: haskell-cafe <<a href="mailto:haskell-cafe@haskell.org" target="_blank">haskell-cafe@haskell.org</a>>
>Subject: Re: [Haskell-cafe] Timing out a pure evaluation of an
> expression I did not write myself
>Message-ID:
> <<a href="mailto:CAJFXAQiDqmsgYJpQB7KRBuiVR5wvwAN-y35r08_Q_TmSkAmyBg@mail.gmail.com" target="_blank">CAJFXAQiDqmsgYJpQB7KRBuiVR5wvwAN-y35r08_Q_TmSkAmyBg@mail.gmail.com</a>>
>Content-Type: text/plain; charset="utf-8"
>
>Actually, after reading the question again, it seems like my response
>wasn't quite right. You are not actually building the list. In that case, I
>am as confused as you. :)
>
>Sorry!
>
>Am So., 18. Nov. 2018 um 01:51 Uhr schrieb Daniel Díaz Casanueva <
><a href="mailto:dhelta.diaz@gmail.com" target="_blank">dhelta.diaz@gmail.com</a>>:
>
>> Hello Ryan.
>>
>> Try evaluating the expression to normal form instead of weak head normal
>> form in your expression. So:
>>
>> >>> timeout 1 $ evaluate $ force $ let x = 0 : x in last x
>>
>> The function `force` comes from the deepseq package. You can read the docs
>> here:
>> <a href="http://hackage.haskell.org/package/deepseq-1.4.4.0/docs/Control-DeepSeq.html" target="_blank">http://hackage.haskell.org/package/deepseq-1.4.4.0/docs/Control-DeepSeq.html</a>
>>
>> I hope that helps.
>>
>> Best regards,
>> Daniel
>>
>> Am So., 18. Nov. 2018 um 00:22 Uhr schrieb Ryan Reich <
>> <a href="mailto:ryan.reich@gmail.com" target="_blank">ryan.reich@gmail.com</a>>:
>>
>>> I want to time out a pure computation. My experience, and that described
>>> in various previous questions here and elsewhere (the best of which is
>>> <a href="https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html" target="_blank">https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html</a>),
>>> is that this doesn't always work: for instance,
>>>
>>> >>> timeout 1 $ evaluate $ let x = 0 : x in last x
>>>
>>> does not time out because, apparently, the fact that the expression
>>> evaluates in constant space (i.e. never allocates) means that it never
>>> yields to the timeout monitor thread that would kill it.
>>>
>>> The solution that is described in the other iterations is to embed
>>> checkpoints in the expression that do allocate, giving the RTS a chance to
>>> switch contexts. However, in my application, the expression is /arbitrary/
>>> and I do not have the freedom to inject alterations into it. (Don't argue
>>> this point, please. The expression is arbitrary.)
>>>
>>> How can I time out a tight loop like the above? Clearly, it can be done,
>>> because I can, say, alt-tab over to another terminal and kill the process,
>>> which exploits the operating system's more aggressively pre-emptive
>>> scheduling. Is there a solution using bound threads, say 'forkOS' instead
>>> of 'forkIO' in the implementation of 'timeout'? Unix signals? Some
>>> FFI-based workaround? Etc. Keep in mind that notwithstanding that
>>> comment, I don't actually want to kill the whole process, but just the one
>>> evaluation.
>>>
>>> Thanks in advance,
>>> Ryan Reich
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> To (un)subscribe, modify options or view archives go to:
>>> <a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe</a>
>>> Only members subscribed via the mailman list are allowed to post.
>>
>>
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>
>------------------------------
>
>Message: 4
>Date: Sat, 17 Nov 2018 16:57:56 -0800
>From: Ryan Reich <<a href="mailto:ryan.reich@gmail.com" target="_blank">ryan.reich@gmail.com</a>>
>To: <a href="mailto:dhelta.diaz@gmail.com" target="_blank">dhelta.diaz@gmail.com</a>
>Cc: haskell-cafe <<a href="mailto:haskell-cafe@haskell.org" target="_blank">haskell-cafe@haskell.org</a>>
>Subject: Re: [Haskell-cafe] Timing out a pure evaluation of an
> expression I did not write myself
>Message-ID:
> <CAO8OckswZrJ2EtvQvfTZOKyi78=<a href="mailto:TqS0dGjgyKYy0fTf38KAgVg@mail.gmail.com" target="_blank">TqS0dGjgyKYy0fTf38KAgVg@mail.gmail.com</a>>
>Content-Type: text/plain; charset="utf-8"
>
>I was just about to reply with an observation to that effect :) The place
>that I'd want to put 'force' is actually inside the 'let' clause, which of
>course you can't do just by applying a function. The expression as a whole
>is just an Integer.
>
>On Sat, Nov 17, 2018 at 4:56 PM Daniel Díaz Casanueva <<a href="mailto:dhelta.diaz@gmail.com" target="_blank">dhelta.diaz@gmail.com</a>>
>wrote:
>
>> Actually, after reading the question again, it seems like my response
>> wasn't quite right. You are not actually building the list. In that case, I
>> am as confused as you. :)
>>
>> Sorry!
>>
>> Am So., 18. Nov. 2018 um 01:51 Uhr schrieb Daniel Díaz Casanueva <
>> <a href="mailto:dhelta.diaz@gmail.com" target="_blank">dhelta.diaz@gmail.com</a>>:
>>
>>> Hello Ryan.
>>>
>>> Try evaluating the expression to normal form instead of weak head normal
>>> form in your expression. So:
>>>
>>> >>> timeout 1 $ evaluate $ force $ let x = 0 : x in last x
>>>
>>> The function `force` comes from the deepseq package. You can read the
>>> docs here:
>>> <a href="http://hackage.haskell.org/package/deepseq-1.4.4.0/docs/Control-DeepSeq.html" target="_blank">http://hackage.haskell.org/package/deepseq-1.4.4.0/docs/Control-DeepSeq.html</a>
>>>
>>> I hope that helps.
>>>
>>> Best regards,
>>> Daniel
>>>
>>> Am So., 18. Nov. 2018 um 00:22 Uhr schrieb Ryan Reich <
>>> <a href="mailto:ryan.reich@gmail.com" target="_blank">ryan.reich@gmail.com</a>>:
>>>
>>>> I want to time out a pure computation. My experience, and that
>>>> described in various previous questions here and elsewhere (the best of
>>>> which is
>>>> <a href="https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html" target="_blank">https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html</a>),
>>>> is that this doesn't always work: for instance,
>>>>
>>>> >>> timeout 1 $ evaluate $ let x = 0 : x in last x
>>>>
>>>> does not time out because, apparently, the fact that the expression
>>>> evaluates in constant space (i.e. never allocates) means that it never
>>>> yields to the timeout monitor thread that would kill it.
>>>>
>>>> The solution that is described in the other iterations is to embed
>>>> checkpoints in the expression that do allocate, giving the RTS a chance to
>>>> switch contexts. However, in my application, the expression is /arbitrary/
>>>> and I do not have the freedom to inject alterations into it. (Don't argue
>>>> this point, please. The expression is arbitrary.)
>>>>
>>>> How can I time out a tight loop like the above? Clearly, it can be
>>>> done, because I can, say, alt-tab over to another terminal and kill the
>>>> process, which exploits the operating system's more aggressively
>>>> pre-emptive scheduling. Is there a solution using bound threads, say
>>>> 'forkOS' instead of 'forkIO' in the implementation of 'timeout'? Unix
>>>> signals? Some FFI-based workaround? Etc. Keep in mind that
>>>> notwithstanding that comment, I don't actually want to kill the whole
>>>> process, but just the one evaluation.
>>>>
>>>> Thanks in advance,
>>>> Ryan Reich
>>>> _______________________________________________
>>>> Haskell-Cafe mailing list
>>>> To (un)subscribe, modify options or view archives go to:
>>>> <a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe</a>
>>>> Only members subscribed via the mailman list are allowed to post.
>>>
>>>
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>------------------------------
>
>Message: 5
>Date: Sun, 18 Nov 2018 12:01:25 +0800 (CST)
>From: ducis <<a href="mailto:ducis_cn@126.com" target="_blank">ducis_cn@126.com</a>>
>To: <a href="mailto:haskell-cafe@haskell.org" target="_blank">haskell-cafe@haskell.org</a>
>Subject: [Haskell-cafe] Specialize a function on types of arguments?
>Message-ID: <<a href="mailto:552773e2.1be5.16724faf3d0.Coremail.ducis_cn@126.com" target="_blank">552773e2.1be5.16724faf3d0.Coremail.ducis_cn@126.com</a>>
>Content-Type: text/plain; charset="gbk"
>
>Hi, everyone,
>
>Is it possible to make combine the following "f" and "g" into one function?
>f:: a -> b -> b
>f x y = y
>g:: a -> a -> a
>g x y = x
>
>Or similarly, "eq1" and "eq2" into one function?
>eq1 :: (Eq a)=>a->a->Bool
>eq1 = (==)
>eq2 :: (Eq a,Eq b)=>a->b->Bool
>eq2 _ _ = False
>
>Looks like it would require some typeclasses, but at least in the first case, "a" and "b" should be any types.
>
>Best!
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>------------------------------
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>Message: 6
>Date: Sun, 18 Nov 2018 17:40:51 +1300
>From: Anthony Clayden <<a href="mailto:anthony_clayden@clear.net.nz" target="_blank">anthony_clayden@clear.net.nz</a>>
>To: <a href="mailto:haskell-cafe@haskell.org" target="_blank">haskell-cafe@haskell.org</a>
>Subject: Re: [Haskell-cafe] Specialize a function on types of
> arguments?
>Message-ID:
> <CAM7nRYRGswE+hepEttL6ccw=<a href="mailto:bKatwBcBqe9fG4VphSFDYOyD1A@mail.gmail.com" target="_blank">bKatwBcBqe9fG4VphSFDYOyD1A@mail.gmail.com</a>>
>Content-Type: text/plain; charset="utf-8"
>
>Hi Ducis,
>
>> Is it possible to make combine the following "f" and "g" into one
>function?
>
>"combine" is vague. You perhaps mean: look at the types of the arguments,
>and choose one function or the other?
>
>> Looks like it would require some typeclasses,
>
>I'll answer the question as put (yes it needs typeclasses), but I can't
>help feel there's a backstory, and you might well be doing something that
>could be done better, if I knew what you're trying to achieve. Let's take
>the second one first
>
>> "eq1" and "eq2" into one function?
>> eq1 :: (Eq a)=>a->a->Bool
>> eq1 = (==)
>> eq2 :: (Eq a,Eq b)=>a->b->Bool
>> eq2 _ _ = False
>
>class Eqbytype a b where
> eqt :: a -> b -> Bool
>
>instance {-# OVERLAPPING #-} (Eq a) => Eqbytype a a where
> eqt = (==)
>
>instance {-# OVERLAPPABLE #-} Eqbytype a b where
> eqt _ _ = False
>
>Look at the Users Guide for what the OVERLAPPING/OVERLAPPABLE pragmas are doing.
>
>Note for the first instance I repeated type var `a` in the head,
>meaning: pick this instance if the two arguments to the method are of
>the same type.
>
>Note for the second instance, I didn't bother with the `Eq`
>constraint, since we can't compare values of distinct types.
>
>
>> f:: a -> b -> b
>> f x y = y
>> g:: a -> a -> a
>> g x y = x
>So you want same argument types to drive which argument to pick? Or
>you want the return type to drive which argument? That's possible:
>look at the definition of class `Read` in the Prelude. Again we can
>pick instances depending on a repeated type. But your requirements are
>not clear.
>
>
>> but at least in the first case [which I've put second], "a" and "b" should be any types.
>
>No they can't: as you state it, you require either all three the same,
>or the second to be the same as the return type.
>
>Come back and ask a more focussed question once you've worked through the
>above. (And explain why you're asking.) The above code is untested, BTW.
>
>AntC
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>------------------------------
>
>Message: 7
>Date: Sun, 18 Nov 2018 09:22:08 +0100
>From: <a href="mailto:arjenvanweelden@gmail.com" target="_blank">arjenvanweelden@gmail.com</a>
>To: <a href="mailto:haskell-cafe@haskell.org" target="_blank">haskell-cafe@haskell.org</a>
>Subject: Re: [Haskell-cafe] Timing out a pure evaluation of an
> expression I did not write myself
>Message-ID: <<a href="mailto:27e9100ff70dd67791ed34e152c58239499bba65.camel@gmail.com" target="_blank">27e9100ff70dd67791ed34e152c58239499bba65.camel@gmail.com</a>>
>Content-Type: text/plain; charset="UTF-8"
>
>On Sat, 2018-11-17 at 15:21 -0800, Ryan Reich wrote:
>> I want to time out a pure computation. My experience, and that
>> described in various previous questions here and elsewhere (the best
>> of which is
>> <a href="https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html" target="_blank">https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html</a>
>> ), is that this doesn't always work: for instance,
>>
>> >>> timeout 1 $ evaluate $ let x = 0 : x in last x
>>
>> does not time out because, apparently, the fact that the expression
>> evaluates in constant space (i.e. never allocates) means that it
>> never yields to the timeout monitor thread that would kill it.
>>
>> The solution that is described in the other iterations is to embed
>> checkpoints in the expression that do allocate, giving the RTS a
>> chance to switch contexts. However, in my application, the
>> expression is /arbitrary/ and I do not have the freedom to inject
>> alterations into it. (Don't argue this point, please. The
>> expression is arbitrary.)
>>
>> How can I time out a tight loop like the above? Clearly, it can be
>> done, because I can, say, alt-tab over to another terminal and kill
>> the process, which exploits the operating system's more aggressively
>> pre-emptive scheduling. Is there a solution using bound threads, say
>> 'forkOS' instead of 'forkIO' in the implementation of 'timeout'?
>> Unix signals? Some FFI-based workaround? Etc. Keep in mind that
>> notwithstanding that comment, I don't actually want to kill the whole
>> process, but just the one evaluation.
>>
>> Thanks in advance,
>> Ryan Reich
>>
>If you are using GHC, the -fno-omit-yields compiler option might be of
>help, which does not optimize out the allocation check that is also
>used for interrupting threads.
>
>See also:
><a href="https://stackoverflow.com/questions/34317730/haskell-timeout-diverging-computation" target="_blank">https://stackoverflow.com/questions/34317730/haskell-timeout-diverging-computation</a>
>
>Are you using the threaded runtime (GHC option -threaded)?
>
>hope this helps, Arjen
>
>
>
>------------------------------
>
>Message: 8
>Date: Sun, 18 Nov 2018 12:53:50 +0100
>From: Fabien R <<a href="mailto:theedge456@free.fr" target="_blank">theedge456@free.fr</a>>
>To: <a href="mailto:haskell-cafe@haskell.org" target="_blank">haskell-cafe@haskell.org</a>
>Subject: [Haskell-cafe] external git dependency source in .cabal
>Message-ID: <<a href="mailto:660c8ca8-3879-b5e5-52c4-682f6e4be80b@free.fr" target="_blank">660c8ca8-3879-b5e5-52c4-682f6e4be80b@free.fr</a>>
>Content-Type: text/plain; charset=utf-8
>
>Hello,
>I'm trying to reference an external source of a package within a sandbox, using cabal <a href="http://2.0.0.1" target="_blank">2.0.0.1</a>:
>
>source-repository head
> type: git
> location: <git URL of pack1>
>
>executable myExe
> build-depends: base==4.10.1.0, pack1 -any
>
>But "cabal -v install --only-dependencies" fails:
>
>cabal: Encountered missing dependencies:
>pack1 -any
>
>Any hint ?
>
>--
>Fabien
>
>
>------------------------------
>
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>
>End of Haskell-Cafe Digest, Vol 183, Issue 14
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