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Today's Topics:
1. Re: Haskeline and forkIO (Jeff C. Britton)
2. Check constructor's field numeric value at compile time
(Dmitriy Matrosov)
3. Re: Check constructor's field numeric value at compile time
(Alexandre Lucchesi)
4. Re: Check constructor's field numeric value at compile time
(Julian Birch)
5. Re: Check constructor's field numeric value at compile time
(Kim-Ee Yeoh)
6. Re: Check constructor's field numeric value at compile time
(Alexandre Lucchesi)
7. Re: Check constructor's field numeric value at compile time
(Brandon Allbery)
----------------------------------------------------------------------
Message: 1
Date: Tue, 2 Sep 2014 18:19:23 +0000
From: "Jeff C. Britton" <[email protected]>
To: Peter Jones <[email protected]>, The Haskell-Beginners Mailing List
- Discussion of primarily beginner-level topics related to Haskell
<[email protected]>
Subject: Re: [Haskell-beginners] Haskeline and forkIO
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="us-ascii"
Thanks Peter,
That suggestion works.
I will have to continue learning more about Monads.
--Jeff
-----Original Message-----
From: Beginners [mailto:[email protected]] On Behalf Of Peter Jones
Sent: Thursday, August 28, 2014 7:11 AM
To: [email protected]
Subject: Re: [Haskell-beginners] Haskeline and forkIO
"Jeff C. Britton" <[email protected]> writes:
> loop :: InputT IO ()
> loop = do
> maybeLine <- getInputLine "Enter a file to compress> "
> case maybeLine of
> Nothing -> return () -- user entered EOF
> Just "" -> return () -- treat no name as "want to quit"
> Just path -> do
> return (runWorker path)
> loop
The other issue you're having is because `runWorker path` is an `IO ()` value
but at the point where you use it in the code the type system wants an `InputT
IO ()`. To try to satisfy the type system you used `return` to build a `InputT
IO (IO ())` value, but that doesn't actually work (as you've noticed). Since
`InputT` is a transformer you have an extra layer to work through and so need
to *lift* your `IO ()` value into the `InputT IO` layer. Try this:
-- Add this import
import Control.Monad.IO.Class
loop :: InputT IO ()
loop = do
maybeLine <- getInputLine "Enter a file to compress> "
case maybeLine of
Nothing -> return () -- user entered EOF
Just "" -> return () -- treat no name as "want to quit"
Just path -> do
liftIO (runWorker path)
loop
You can think of `liftIO` as having this signature (in this context):
liftIO :: IO () -> InputT IO ()
--
Peter Jones, Founder, Devalot.com
Defending the honor of good code
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------------------------------
Message: 2
Date: Tue, 02 Sep 2014 23:32:35 +0400
From: Dmitriy Matrosov <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Check constructor's field numeric value
at compile time
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hi.
I have a type
data Interval = Seconds Float
| MicroSeconds Int
The Float field of data constructor Seconds should be >= 1, and Int
field of constructor MicroSeconds should be in the range from 0 to 1000000.
How can i write this constraints so they're checked at compile time, not
at runtime?
--
Dmitriy Matrosov
------------------------------
Message: 3
Date: Tue, 2 Sep 2014 17:58:24 -0300
From: Alexandre Lucchesi <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Check constructor's field numeric
value at compile time
Message-ID:
<cagqbpevraeccsseqqw1m4cyrv9o2j+g0bxcxzfstsj7xsqm...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
You can't do that within the data type declaration.
In order to add such constraints you should define "constructor functions"
and apply the validation there, i.e.:
newSecondsInterval :: Float -> Maybe Interval
newSecondsInterval n
| n >= 1 = Just $ Seconds n
| otherwise = Nothing
Note that the "Maybe" type is only a way to handle the case where the
supplied value is invalid. Also, in order to provide encapsulation and
ensure no one is going to create a new "Interval" by using "Seconds" and
"MicroSeconds", you should hide these value constructors in the export list
of your module.
On Tue, Sep 2, 2014 at 4:32 PM, Dmitriy Matrosov <[email protected]> wrote:
> Hi.
>
> I have a type
>
> data Interval = Seconds Float
> | MicroSeconds Int
>
> The Float field of data constructor Seconds should be >= 1, and Int field
> of constructor MicroSeconds should be in the range from 0 to 1000000.
>
> How can i write this constraints so they're checked at compile time, not
> at runtime?
>
> --
> Dmitriy Matrosov
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
--
alexandre lucchesi
*Perfection is achieved, not when there is nothing more to add, but when
there is nothing left to take away!*
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Message: 4
Date: Tue, 2 Sep 2014 23:02:48 +0100
From: Julian Birch <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Check constructor's field numeric
value at compile time
Message-ID:
<cab0tuzcbwmy5xpv4uqqszuzqgtplq6ut8azdca38hzbimjm...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Also, if you want your mind blown, check out Idris.
On Tuesday, September 2, 2014, Alexandre Lucchesi <
[email protected]> wrote:
> You can't do that within the data type declaration.
>
> In order to add such constraints you should define "constructor functions"
> and apply the validation there, i.e.:
>
> newSecondsInterval :: Float -> Maybe Interval
> newSecondsInterval n
> | n >= 1 = Just $ Seconds n
> | otherwise = Nothing
>
> Note that the "Maybe" type is only a way to handle the case where the
> supplied value is invalid. Also, in order to provide encapsulation and
> ensure no one is going to create a new "Interval" by using "Seconds" and
> "MicroSeconds", you should hide these value constructors in the export list
> of your module.
>
>
> On Tue, Sep 2, 2014 at 4:32 PM, Dmitriy Matrosov <[email protected]
> <javascript:_e(%7B%7D,'cvml','[email protected]');>> wrote:
>
>> Hi.
>>
>> I have a type
>>
>> data Interval = Seconds Float
>> | MicroSeconds Int
>>
>> The Float field of data constructor Seconds should be >= 1, and Int field
>> of constructor MicroSeconds should be in the range from 0 to 1000000.
>>
>> How can i write this constraints so they're checked at compile time, not
>> at runtime?
>>
>> --
>> Dmitriy Matrosov
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> <javascript:_e(%7B%7D,'cvml','[email protected]');>
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>
>
>
> --
> alexandre lucchesi
>
> *Perfection is achieved, not when there is nothing more to add, but when
> there is nothing left to take away!*
>
--
Sent from an iPhone, please excuse brevity and typos.
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Message: 5
Date: Wed, 3 Sep 2014 10:04:10 +0700
From: Kim-Ee Yeoh <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Check constructor's field numeric
value at compile time
Message-ID:
<CAPY+ZdRcp1=_GDXrGOt5OtkwD-1ndk+=nsyh82laa88kamp...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Wed, Sep 3, 2014 at 3:58 AM, Alexandre Lucchesi <
[email protected]> wrote:
> In order to add such constraints you should define "constructor functions"
> and apply the validation there, i.e.:
To OP: Yes, the searchable term is "smart constructors". However, it has
been used at least once on this list to mean something different, so look
out.
-- Kim-Ee
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Message: 6
Date: Wed, 3 Sep 2014 00:07:10 -0300
From: Alexandre Lucchesi <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Check constructor's field numeric
value at compile time
Message-ID:
<CAGqBpeUaPeUPYWKjhjRqdHYEnt-s=48uzgqTnVpqN=smhqc...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hmm.. Nice to know about that!
Thank you. :)
On Wed, Sep 3, 2014 at 12:04 AM, Kim-Ee Yeoh <[email protected]> wrote:
>
> On Wed, Sep 3, 2014 at 3:58 AM, Alexandre Lucchesi <
> [email protected]> wrote:
>
>> In order to add such constraints you should define "constructor
>> functions" and apply the validation there, i.e.:
>
>
> To OP: Yes, the searchable term is "smart constructors". However, it has
> been used at least once on this list to mean something different, so look
> out.
>
> -- Kim-Ee
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
--
alexandre lucchesi
*Perfection is achieved, not when there is nothing more to add, but when
there is nothing left to take away!*
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Message: 7
Date: Tue, 2 Sep 2014 23:07:31 -0400
From: Brandon Allbery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Check constructor's field numeric
value at compile time
Message-ID:
<cakfcl4we5vdavt0fjkxlpsytuq-vw4-jw-nf7jjvbgyy1to...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Tue, Sep 2, 2014 at 11:04 PM, Kim-Ee Yeoh <[email protected]> wrote:
>
> On Wed, Sep 3, 2014 at 3:58 AM, Alexandre Lucchesi <
> [email protected]> wrote:
>
>> In order to add such constraints you should define "constructor
>> functions" and apply the validation there, i.e.:
>
>
> To OP: Yes, the searchable term is "smart constructors". However, it has
> been used at least once on this list to mean something different, so look
> out.
>
Also note that this will not actually check at compile time; to do that you
need type level stuff that ghc is still finding its way through.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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End of Beginners Digest, Vol 75, Issue 2
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