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Today's Topics:
1. Re: How to call popCnt64#? (John Ky)
2. Re: How to call popCnt64#? (Marcin Mrotek)
3. Need help groking the statement: expression1 in
term:expression2 (Olumide)
4. Do IO actions *have* to be glued by the do syntax? (Olumide)
5. Re: Need help groking the statement: expression1 in
term:expression2 (Quanyang Liu)
6. Re: Do IO actions *have* to be glued by the do syntax?
(Quanyang Liu)
7. Re: Do IO actions *have* to be glued by the do syntax?
(Daniel Wright)
----------------------------------------------------------------------
Message: 1
Date: Mon, 21 Mar 2016 22:30:17 +0000
From: John Ky <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] How to call popCnt64#?
Message-ID:
<CAMB4o-BugzHt=7mWFLodApGA=opi28wdewj8exepz-nynua...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi Michael,
Yes, that did the trick.
Thanks!
-John
On Mon, 21 Mar 2016 at 01:13 Michael Snoyman <[email protected]> wrote:
> Here's an example:
>
> {-# LANGUAGE MagicHash #-}
> import GHC.Prim
> import GHC.Types
> import Data.Word
>
> main :: IO ()
> main = do
> let word = 5 :: Word
> res =
> case word of
> W# w -> W# (popCnt64# w)
> print res
>
>
> On Sun, Mar 20, 2016 at 12:19 PM, John Ky <[email protected]> wrote:
>
>> Hello Haskellers,
>>
>> Does anyone know how to call popCnt64# from the GHC.Prim module?
>>
>> This was my failed attempt:
>>
>> ?> popCnt64# 1
>>
>>
>> <interactive>:14:11:
>> Couldn't match kind ?*? with ?#?
>> When matching types
>> a0 :: *
>> Word# :: #
>> Expected type: Integer -> Word#
>> Actual type: Integer -> a0
>> In the first argument of ?popCnt64#?, namely ?1?
>> In the expression: popCnt64# 1
>> In an equation for ?it?: it = popCnt64# 1
>>
>> -John
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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Message: 2
Date: Tue, 22 Mar 2016 08:25:15 +0100
From: Marcin Mrotek <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] How to call popCnt64#?
Message-ID:
<CAJcfPzmrgQgu85FVMit0POEXLOJ43Lz_u1=ppezo2ee4nku...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi,
In general, the problem is that GHCi is attempting to call `show` on the
results of expressions you type in, but `show` (like any other polymorphic
function; though you can look into "levity polymorphism" if you want to
know more) can only accept values of types of kind * (boxed, lifted) - so
it can print Word, but not Word#.
If you wanted to stay in GHCi, you can do it like:
Prelude> import GHC.Prim
Prelude GHC.Prim> import GHC.Types
Prelude GHC.Prim GHC.Types> :set -XMagicHash
Prelude GHC.Prim GHC.Types> :t W#
W# :: Word# -> Word
Prelude GHC.Prim GHC.Types> :t popCnt64#
popCnt64# :: Word# -> Word#
Prelude GHC.Prim GHC.Types> let foo = (1 :: Word)
Prelude GHC.Prim GHC.Types> :set -XBangPatterns
Prelude GHC.Prim GHC.Types> let !(W# w) = foo in W# (popCnt64# w)
1
Best regards,
Marcin Mrotek
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Message: 3
Date: Tue, 22 Mar 2016 10:43:46 +0000
From: Olumide <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Need help groking the statement:
expression1 in term:expression2
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
Hello List,
I'd appreciate help understanding the second line of following block of
code (from LYH, first line added for completeness),
http://learnyouahaskell.com/input-and-output#randomness
randoms' :: (RandomGen g, Random a) => g -> [a]
randoms' gen = let (value, newGen) = random gen in value:randoms' newGen
The part I'm really struggling with is random gen in value:randoms' newGen
Thanks,
- Olumide
------------------------------
Message: 4
Date: Tue, 22 Mar 2016 10:52:41 +0000
From: Olumide <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Do IO actions *have* to be glued by the
do syntax?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
Hello List,
Do IO actions *have* to be glued by the do syntax? Many, if not all, the
examples that I've come across in LYH seem to suggest so. And if so, why?
BTW, if possible I'd appreciate an explanation that does not resort to
monads. I haven't studied them yet and I'm sure I'd struggle to
understand any explanation that resorts to monads.
Thanks,
- Olumide
------------------------------
Message: 5
Date: Tue, 22 Mar 2016 18:57:56 +0800
From: Quanyang Liu <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Need help groking the statement:
expression1 in term:expression2
Message-ID: <[email protected]>
Content-Type: text/plain
On Tue, Mar 22 2016 at 18:43:46 +0800, Olumide wrote:
> Hello List,
>
> I'd appreciate help understanding the second line of following block
> of code (from LYH, first line added for completeness),
>
> http://learnyouahaskell.com/input-and-output#randomness
> randoms' :: (RandomGen g, Random a) => g -> [a]
> randoms' gen = let (value, newGen) = random gen in value:randoms' newGen
>
It's equivalent to the following:
randoms' gen = let (value, newGen) = random gen
in value:(randoms' newGen)
> The part I'm really struggling with is random gen in value:randoms' newGen
>
> Thanks,
>
> - Olumide
--
Thanks,
Quanyang
------------------------------
Message: 6
Date: Tue, 22 Mar 2016 19:06:21 +0800
From: Quanyang Liu <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Do IO actions *have* to be glued by
the do syntax?
Message-ID: <[email protected]>
Content-Type: text/plain
On Tue, Mar 22 2016 at 18:52:41 +0800, Olumide wrote:
> Hello List,
>
> Do IO actions *have* to be glued by the do syntax? Many, if not all,
> the examples that I've come across in LYH seem to suggest so. And if
> so, why?
>
> BTW, if possible I'd appreciate an explanation that does not resort to
> monads. I haven't studied them yet and I'm sure I'd struggle to
> understand any explanation that resorts to monads.
>
I think you should google that first... do blocks are just sugar...
> Thanks,
>
> - Olumide
--
Thanks,
Quanyang
------------------------------
Message: 7
Date: Tue, 22 Mar 2016 20:26:00 +0900
From: Daniel Wright <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Do IO actions *have* to be glued by
the do syntax?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Hello Olumide,
The simple answer is "no", do syntax is an entirely optional piece of syntactic
sugar that is just there to make the code easier to read in some cases. But,
IO actions do have to be glued together by something, and that something is the
>>= operator (pronounced "bind").
>>= takes an IO action on the left, (let's use getLine, which reads a line from
>>the user, for our example) and a function on the right which takes the result
>>of that action and returns the next IO action to perform. For example:
> getLine >>= (\line -> putStrLn line)
Here we can see the getLine action on the left. The result of that is passed,
through the mechanism provided by >>=, to the function on the right, which is a
lambda function taking that line as a parameter and calling the action
"putStrLn" to print it back out again. This is equivalent to the following do
notation:
> do
> line <- getLine
> putStrLn line
What if you want to string together more than two actions, though? For
example, say we wanted to get two lines and concatenation them in the output,
like so:
> do
> line1 <- getLine
> line2 <- getLine
> putStrLn (line1 ++ line2)
Since >>= can only take two parameters, what do we do? Well, since the result
of >>= is itself an IO action, we can use it again in the function on the
right! Like this:
> getLine >>= (\line1 -> (getLine >>= (\line2 -> putStrLn (line1 ++ line2))))
Now, this may be starting to look a little bit like lisp with all the
parentheses! In fact, because of the associativity of the >>= operator, the
brackets are all optional, but they may be helpful for you to see what's going
on. Without the brackets, it looks like this:
> getLine >>= \line1 -> getLine >>= \line2 -> putStrLn (line1 ++ line2)
Which, if you were to lay it out a little differently, looks like this:
> getLine >>= \line1 ->
> getLine >>= \line2 ->
> putStrLn (line1 ++ line2)
...Starting to look a little more like the do syntax version above, isn't it?
What if you don't care about the value of the parameter? Well, you could
ignore it using the _ syntax like this:
> getLine >>= \_ -> putStrLn "42"
But Haskell actually provides an operator to save you the effort, called >>, so
you can do:
> getLine >> putStrLn "42"
Do notation is a purely syntactic transformation that emits code using these
operators.
Incidentally, the function on the right doesn't have to be a lambda function!
Any function taking a value of whatever type is returned by the action on the
left will do, and in the case of getLine/putStrLn they match! So the first
example could be rewritten:
> getLine >>= putStrLn
Which many would argue looks nicer than the equivalent do notation.
Hope that helps, sorry if it was a little verbose!
-Dani.
PS- I know you said not to mention monads, but if you followed everything above
you're 90% of the way there anyway -- a monad is just a type class providing
the >>= operator, and the "return" function, which takes a value and turns it
into an action returning that value (at least in terms of the IO monad).
> On Mar 22, 2016, at 19:52, Olumide <[email protected]> wrote:
>
> Hello List,
>
> Do IO actions *have* to be glued by the do syntax? Many, if not all, the
> examples that I've come across in LYH seem to suggest so. And if so, why?
>
> BTW, if possible I'd appreciate an explanation that does not resort to
> monads. I haven't studied them yet and I'm sure I'd struggle to understand
> any explanation that resorts to monads.
>
> Thanks,
>
> - Olumide
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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