[Haskell-cafe] CDouble type coercion

[SevenThunders]

I am new to Haskell and found myself in a bind concerning the use of 
the C types, CDouble in particular.  I extract a CDouble via it's pointer
from a StorableArray.  Since the array must interface with C the elements of
the array must be CDouble.  Now I'd like to use Text.Printf to format print
statements of elements of the array, but Text.Printf requires Doubles as
inputs and so far I have not found an obvious way to coerce CDoubles into
Doubles.

Additionally,  withStorableArray returns a monad containing a CDouble
instead of Double as peek I suppose does.  Because I cant coerce my CDouble
into a Double,  printf chokes.  Thus the following code fails to compile on
GHC

import Foreign
import Foreign.C
import Foreign.C.Types (CInt, CDouble )
import Data.Array.Storable
import Text.Printf


foreign import ccall matrix_c.h sumarr sumarr :: Ptr CDouble - IO
(CDouble)

main = do
arr - newListArray (1 , 3) [3,2,1]:: IO (StorableArray Int CDouble)
-- extract the pointer to arr
withStorableArray arr sumarr = (\x - printf 15.7f\n x)


The error message is,
test2.hs:13:44:
No instance for (PrintfArg CDouble)
  arising from use of `printf' at test2.hs:13:44-49
Probable fix: add an instance declaration for (PrintfArg CDouble)
In a lambda abstraction: \ x - printf 15.7f\n x
In the second argument of `(=)', namely `(\ x - printf 15.7f\n x)'
In the result of a 'do' expression:
(withStorableArray arr sumarr) = (\ x - printf 15.7f\n x)



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Re: [Haskell-cafe] CDouble type coercion

[Sven Panne]

Am Sonntag, 14. Mai 2006 09:30 schrieb SevenThunders:
 I am new to Haskell and found myself in a bind concerning the use of
 the C types, CDouble in particular.  I extract a CDouble via it's pointer
 from a StorableArray.  Since the array must interface with C the elements
 of the array must be CDouble.  Now I'd like to use Text.Printf to format
 print statements of elements of the array, but Text.Printf requires Doubles
 as inputs and so far I have not found an obvious way to coerce CDoubles
 into Doubles. [...]

You can use the Prelude function realToFrac to convert between the various 
floating-point types:


[EMAIL PROTECTED]:~ ghci -v0
Prelude :t realToFrac
realToFrac :: (Fractional t1, Real t) = t - t1
Prelude (realToFrac :: Foreign.C.Types.CDouble - Double) 1234.5
1234.5


As you can see from its type, realToFrac is not exactly about floating-point 
conversions, but for almost all practical use cases it is. :-)

Cheers,
   S.
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Re: [Haskell-cafe] CDouble type coercion

[SevenThunders]

Thanks that helps a bit.  The realToFrac type coercion works, but ultimately
it seems 
that printf won't play nice.  Consider this simple haskell code

module Test
where
import IO
-- import Data.Array.Storable 
import Text.Printf
import Foreign.C.Types (CInt, CDouble )

y :: CDouble
y = 5.2
u = realToFrac(y)
test = do  printf %14.7g u   

Compiling it into GHCi I get the error,
test.hs:13:11:
No instance for (PrintfType (t t1))
  arising from use of `printf' at test.hs:13:11-16
Probable fix: add an instance declaration for (PrintfType (t t1))
In the result of a 'do' expression: printf %14.7g u
In the definition of `test': test = do printf %14.7g u
Failed, modules loaded: none.

If I replace the printf by a standard print there is no problem.  Also if I
comment out
the line with test = ... , it will load (compile) and I can then type in the
interpreter
printf %14.7g u 
and it works!

but
printf %14.7g y

fails because y is of type CDouble.  At this point I think I'm giving up on
the 
formatted printing in Haskell.  It's just too persnickety. 
I guess I'll just call printf via C and see what happens.

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Re: [Haskell-cafe] CDouble type coercion

[Robert Dockins]

On Sunday 14 May 2006 03:00 pm, SevenThunders wrote:
 Thanks that helps a bit.  The realToFrac type coercion works, but
 ultimately it seems
 that printf won't play nice.  Consider this simple haskell code

 module Test
   where
 import IO
 -- import Data.Array.Storable
 import Text.Printf
 import Foreign.C.Types (CInt, CDouble )

 y :: CDouble
 y = 5.2
 u = realToFrac(y)
 test = do  printf %14.7g u

Try:

y :: CDouble
y = 5.2
u :: Double
u = realToFrac(y)
test = do  printf %14.7g u

The root problem seems to be that GHC isn't sure what type 'u' has unless you 
fix it with a type signature because 'realToFrac' has a polymorphic type.  
I'm not sure why it works when you type it at the interpreter.


 Compiling it into GHCi I get the error,
 test.hs:13:11:
 No instance for (PrintfType (t t1))
   arising from use of `printf' at test.hs:13:11-16
 Probable fix: add an instance declaration for (PrintfType (t t1))
 In the result of a 'do' expression: printf %14.7g u
 In the definition of `test': test = do printf %14.7g u
 Failed, modules loaded: none.

 If I replace the printf by a standard print there is no problem.  Also if I
 comment out
 the line with test = ... , it will load (compile) and I can then type in
 the interpreter
 printf %14.7g u
 and it works!

 but
 printf %14.7g y

 fails because y is of type CDouble.  At this point I think I'm giving up on
 the
 formatted printing in Haskell.  It's just too persnickety.
 I guess I'll just call printf via C and see what happens.

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Re: [Haskell-cafe] CDouble type coercion

[SevenThunders]

The rabbit hole goes a bit deeper I'm afraid
y :: CDouble
y = 5.2
u :: Double
u = realToFrac(y)
test = do  printf %14.7g u 

gives
Compiling Test ( test.hs, interpreted )

test.hs:14:11:
No instance for (PrintfType (t t1))
  arising from use of `printf' at test.hs:14:11-16
Probable fix: add an instance declaration for (PrintfType (t t1))
In the result of a 'do' expression: printf %14.7g u
In the definition of `test': test = do printf %14.7g u
Failed, modules loaded: none.

But in fact just this, gives the same error
test = do  printf %14.7g 3.14 

Now in the command line I get errors under various configurations but a few
of them 
work e.g.
works
 printf %g u
 printf %g (3.14 :: Double)
do printf %g u
do printf %g (3.14 :: Double)

fails
printf %g y -- y is  a CDouble
printf %g 3.14


All forms fail when compiled from a .hs file.  However, this just in!  I
finally got something to work,
namely this piece of code
test = do  (printf %14.7g (u  :: Double)) :: IO()

I guess the output of printf has to be disambiguated.  There should probably
be a few examples of this in the library documentation.  This also works, by
returning a string...
test = printf %14.7g (u  :: Double) :: String 

Thanks for the help by the way, without realtoFrac there would be no way for
me to use this at all.
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Re: [Haskell-cafe] CDouble type coercion

[Twan van Laarhoven]

SevenThunders wrote:
 test.hs:14:11:
 No instance for (PrintfType (t t1))
   arising from use of `printf' at test.hs:14:11-16
 Probable fix: add an instance declaration for (PrintfType (t t1))
 In the result of a 'do' expression: printf %14.7g u
 In the definition of `test': test = do printf %14.7g u
 Failed, modules loaded: none.

The problem here appears to be the monomorphism restriction. This means 
that all declarations without arguments get a monomorphic type, a type 
without variables. The type of 'test' would be 'PrintfType r = r', but 
the compiler does not allow this type. The solution is to either:
 - make the type explicit by adding a type declaration, test :: IO () 
or Test :: PrintfType r = r
 - make test local in a context where it is only used as an IO action, 
for example:

main :: IO ()
main = do {... ; test ; ...}
  where test = printf %14.7g 3.14
 - add a parameter to test

Twan
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