Re: [Haskell-cafe] Casting newtype to base type?

2013-07-06 Thread Ozgur Akgun 
Hi Vlatko.

On 2 July 2013 16:03, Vlatko Basic vlatko.ba...@gmail.com wrote:

 Is there a nicer way to extract the 'IO String' from 'IOS',
 without 'case' or without pattern matching the whole 'P'?


You might enjoy the newtype package.

http://hackage.haskell.org/package/newtype

Hope this helps,
Ozgur.
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Re: [Haskell-cafe] Casting newtype to base type?

2013-07-02 Thread Vlatko Basic 

Is there a nicer way to extract the 'IO String' from 'IOS',
without 'case' or without pattern matching the whole 'P'?

newtype IOS = IOS (IO String)
data P = P {
  getA :: String,
  getB :: String,
  getC :: IOS
  } deriving (Show, Eq)


getC_IO :: P - IO String
getC_IO p =
  case getC p of
IOS a - a
getC_IO (P _ _ (IOS a)) = a



 Original Message  
Subject: Re: [Haskell-cafe] Casting newtype to base type?
From: Malcolm Wallace malcolm.wall...@me.com
To: vlatko.ba...@gmail.com
Cc: Haskell-Cafe haskell-cafe@haskell.org
Date: 01.07.2013 17:24



On 1 Jul 2013, at 16:07, Vlatko Basic wrote:


I had a (simplified) record

  data P = P {
a :: String,
b :: String,
c :: IO String
} deriving (Show, Eq)

but to get automatic deriving of 'Show' and 'Eq' for 'data P' I have created 
'newtype IOS' and its 'Show' and 'Eq' instances

  newtype IOS = IO String


Not quite!  That is a newtype'd String, not a newtype's (IO String).  Try this:

 newtype IOS = IOS (IO String)


but now when I try to set 'c' field in

return $ p {c = readFile path}

I get error
  Couldn't match expected type `IOS' with actual type `IO String'


Use the newtype constructor to convert an IO String - IOS.

 return $ p {c = IOS $ readFile path}

Regards,
 Malcolm



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Re: [Haskell-cafe] Casting newtype to base type?

2013-07-02 Thread Tom Ellis 
On Tue, Jul 02, 2013 at 03:03:08PM +0200, Vlatko Basic wrote:
 Is there a nicer way to extract the 'IO String' from 'IOS',
 without 'case' or without pattern matching the whole 'P'?
 
 newtype IOS = IOS (IO String)
 data P = P {
   getA :: String,
   getB :: String,
   getC :: IOS
   } deriving (Show, Eq)
 
 
 getC_IO :: P - IO String
 getC_IO p =
   case getC p of
 IOS a - a
 getC_IO (P _ _ (IOS a)) = a

How about

unIOS :: IOS - IO String
unIOS (IOS a) = a

getC_IO :: P - IO String
getC_IO = unIOS . getC

Tom

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Re: [Haskell-cafe] Casting newtype to base type?

2013-07-02 Thread Vlatko Basic 



 Original Message  
Subject: Re: [Haskell-cafe] Casting newtype to base type?
From: Tom Ellis tom-lists-haskell-cafe-2...@jaguarpaw.co.uk
To: haskell-cafe@haskell.org
Date: 02.07.2013 15:25


On Tue, Jul 02, 2013 at 03:03:08PM +0200, Vlatko Basic wrote:

Is there a nicer way to extract the 'IO String' from 'IOS',
without 'case' or without pattern matching the whole 'P'?

newtype IOS = IOS (IO String)
data P = P {
   getA :: String,
   getB :: String,
   getC :: IOS
   } deriving (Show, Eq)


getC_IO :: P - IO String
getC_IO p =
   case getC p of
 IOS a - a
getC_IO (P _ _ (IOS a)) = a


How about

 unIOS :: IOS - IO String
 unIOS (IOS a) = a

 getC_IO :: P - IO String
 getC_IO = unIOS . getC


Thanks for your answer.
I had those two funcs, but thought there might be a shorter/prettier one-func 
one-liner. :-)



Tom

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Re: [Haskell-cafe] Casting newtype to base type?

2013-07-02 Thread David McBride 
You could always just put it into your newtype:

newtype IOS = IOS {
  unIOS :: IO String
}

On Tue, Jul 2, 2013 at 9:31 AM, Vlatko Basic vlatko.ba...@gmail.com wrote:


  Original Message  
 Subject: Re: [Haskell-cafe] Casting newtype to base type?
 From: Tom Ellis tom-lists-haskell-cafe-2...@jaguarpaw.co.uk
 To: haskell-cafe@haskell.org
 Date: 02.07.2013 15:25

 On Tue, Jul 02, 2013 at 03:03:08PM +0200, Vlatko Basic wrote:

 Is there a nicer way to extract the 'IO String' from 'IOS',
 without 'case' or without pattern matching the whole 'P'?

 newtype IOS = IOS (IO String)
 data P = P {
getA :: String,
getB :: String,
getC :: IOS
} deriving (Show, Eq)


 getC_IO :: P - IO String
 getC_IO p =
case getC p of
  IOS a - a
 getC_IO (P _ _ (IOS a)) = a


 How about

  unIOS :: IOS - IO String
  unIOS (IOS a) = a

  getC_IO :: P - IO String
  getC_IO = unIOS . getC

 Thanks for your answer.
 I had those two funcs, but thought there might be a shorter/prettier
 one-func one-liner. :-)


 Tom

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Re: [Haskell-cafe] Casting newtype to base type?

2013-07-01 Thread Frerich Raabe 

Am 7/1/2013 5:07 PM, schrieb Vlatko Basic:

to get automatic deriving of 'Show' and 'Eq' for 'data P' I have
created 'newtype IOS' and its 'Show' and 'Eq' instances

   newtype IOS = IO String


What you really want is

  newtype IOS = IOS (IO String)

I.e. a IOS value wraps an IO String.


   data P = P {
 a :: String,
 b :: String,
 c :: IOS
 } deriving (Show, Eq)

but now when I try to set 'c' field in

   fun :: FilePath - P - IO P
   fun path p = do
 b - doesFileExist path
 ...
 return $ p {c = readFile path}

I get error
   Couldn't match expected type `IOS' with actual type `IO String'

which is correct.

So, the question is:

   Is it possible to somehow cast 'IO String' to 'IOS'


With the change to your 'newtype', you could use

  return $ p {c = IOS (readFile path)}

instead.

--
Frerich Raabe - ra...@froglogic.com
www.froglogic.com - Multi-Platform GUI Testing


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Re: [Haskell-cafe] Casting newtype to base type?

2013-07-01 Thread Tom Ellis 
On Mon, Jul 01, 2013 at 05:07:00PM +0200, Vlatko Basic wrote:
 Hello Cafe!
 
 I had a (simplified) record
 
   data P = P {
 a :: String,
 b :: String,
 c :: IO String
 } deriving (Show, Eq)
 
 but to get automatic deriving of 'Show' and 'Eq' for 'data P' I have
 created 'newtype IOS' and its 'Show' and 'Eq' instances
 
   newtype IOS = IO String
   instance Show (IOS) where show _ = (IO String) function
   instance Eq   (IOS) where _ == _ = True

An Eq instance for something containing IO is bound to lead to puzzlement
somewhere down the line.  I think you're better off defining something like

data P_lesser = P_lesser {
a_lesser :: String,
b_lesser :: String
} deriving (Show, Eq)

to_lesser p = P_lesser (a p) (b p)

and just factoring everything through to_lesser when you want to compare
or show.

Tom

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Re: [Haskell-cafe] Casting newtype to base type?

2013-07-01 Thread Malcolm Wallace 

On 1 Jul 2013, at 16:07, Vlatko Basic wrote:

 I had a (simplified) record
 
  data P = P {
a :: String,
b :: String,
c :: IO String
} deriving (Show, Eq)
 
 but to get automatic deriving of 'Show' and 'Eq' for 'data P' I have created 
 'newtype IOS' and its 'Show' and 'Eq' instances
 
  newtype IOS = IO String

Not quite!  That is a newtype'd String, not a newtype's (IO String).  Try this:

newtype IOS = IOS (IO String)

 but now when I try to set 'c' field in
 
return $ p {c = readFile path}
 
 I get error
  Couldn't match expected type `IOS' with actual type `IO String'

Use the newtype constructor to convert an IO String - IOS.

return $ p {c = IOS $ readFile path}

Regards,
Malcolm
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Re: [Haskell-cafe] Casting newtype to base type?

2013-07-01 Thread Vlatko Basic 

Hi Wallace,

yes, indeed. Now I see I mixed newtype with type in declaration, and forgot the 
first one is a constructor.



 Original Message  
Subject: Re: [Haskell-cafe] Casting newtype to base type?
From: Malcolm Wallace malcolm.wall...@me.com
To: vlatko.ba...@gmail.com
Cc: Haskell-Cafe haskell-cafe@haskell.org
Date: 01.07.2013 17:24



On 1 Jul 2013, at 16:07, Vlatko Basic wrote:


I had a (simplified) record

  data P = P {
a :: String,
b :: String,
c :: IO String
} deriving (Show, Eq)

but to get automatic deriving of 'Show' and 'Eq' for 'data P' I have created 
'newtype IOS' and its 'Show' and 'Eq' instances

  newtype IOS = IO String


Not quite!  That is a newtype'd String, not a newtype's (IO String).  Try this:

 newtype IOS = IOS (IO String)


but now when I try to set 'c' field in

return $ p {c = readFile path}

I get error
  Couldn't match expected type `IOS' with actual type `IO String'


Use the newtype constructor to convert an IO String - IOS.

 return $ p {c = IOS $ readFile path}

Regards,
 Malcolm



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Re: [Haskell-cafe] Casting newtype to base type?

2013-07-01 Thread Vlatko Basic 
I'm experimenting. The IO field is just a helper field, so it shouldn't have any 
consequences.



 Original Message  
Subject: Re: [Haskell-cafe] Casting newtype to base type?
From: Tom Ellis tom-lists-haskell-cafe-2...@jaguarpaw.co.uk
To: haskell-cafe@haskell.org
Date: 01.07.2013 17:24


On Mon, Jul 01, 2013 at 05:07:00PM +0200, Vlatko Basic wrote:

Hello Cafe!

I had a (simplified) record

   data P = P {
 a :: String,
 b :: String,
 c :: IO String
 } deriving (Show, Eq)

but to get automatic deriving of 'Show' and 'Eq' for 'data P' I have
created 'newtype IOS' and its 'Show' and 'Eq' instances

   newtype IOS = IO String
   instance Show (IOS) where show _ = (IO String) function
   instance Eq   (IOS) where _ == _ = True


An Eq instance for something containing IO is bound to lead to puzzlement
somewhere down the line.  I think you're better off defining something like

 data P_lesser = P_lesser {
 a_lesser :: String,
 b_lesser :: String
 } deriving (Show, Eq)

 to_lesser p = P_lesser (a p) (b p)

and just factoring everything through to_lesser when you want to compare
or show.

Tom

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