Lets suppose I have a file that has encoded things of different types as integers, and now I would like to convert them back into specific instances of a data type. For example, I have a file that contains 1,1,2,3 and I would like the output to be [Red, Red, Green, Blue]. I also would like to do this generically, so that if I wanted to convert the same list of integers into say Sizes, I would get [Small, Small, Medium, Large] Now please have a look at the following code: {-# LANGUAGE DeriveDataTypeable #-} import Data.Generics data Color = Red | Green | Blue deriving (Eq,Ord,Read,Show,Typeable,Data) data Size = Small | Mediaum | Large deriving (Eq,Ord,Read,Show,Typeable,Data) g = Green c = undefined :: Color s = undefined :: Size t = do print $ toConstr g -- Green print $ dataTypeOf c -- DataType {tycon = "Main.Color", datarep = AlgRep [Red,Green,Blue]} convert :: (Data a, Data b) =Int -a -b convert i x = let c = dataTypeConstrs (dataTypeOf x) !! (i-1) in fromConstr c I would like to be able to say: x = convert 1 c and have it assign Red to x then I would like to say: y = convert 1 s and have it assign Small to y, however, when I try that I get: Ambiguous type variable `b' in the constraint: `Data b' arising from a use of `convert' at <interactive>:1:8-18 Probable fix: add a type signature that fixes these type variable(s) Of course if I say x :: Color = convert 1 c, it works, but I would like to avoid that if possible, as all of the information is already contained in the parameter c. Is there any way to do this? Thanks in advance for your wise counsel. Best wishes, Henry Laxen
On Mon, Jun 8, 2009 at 4:10 PM, Henry Laxen
Lets suppose I have a file that has encoded things of different types as integers, and now I would like to convert them back into specific instances of a data type. For example, I have a file that contains 1,1,2,3 and I would like the output to be [Red, Red, Green, Blue]. I also would like to do this generically, so that if I wanted to convert the same list of integers into say Sizes, I would get [Small, Small, Medium, Large] Now please have a look at the following code:
{-# LANGUAGE DeriveDataTypeable #-} import Data.Generics data Color = Red | Green | Blue deriving (Eq,Ord,Read,Show,Typeable,Data) data Size = Small | Mediaum | Large deriving (Eq,Ord,Read,Show,Typeable,Data)
What about making both of these instances of Enum instead of using Data and Typeable? You'd get fromEnum and toEnum. Which I think, would give you the int mapping that you are after. fromEnum :: Enum a => a -> Int toEnum :: Enum a => Int -> a Jason
Henry, Jason pointed out:
You'd get fromEnum and toEnum. Which I think, would give you the int mapping that you are after.
fromEnum :: Enum a => a -> Int toEnum :: Enum a => Int -> a
To me, this would indeed seem the way to go for your particular example. Moreover, as for generic producer functions in general, the pattern suggested by the Prelude would be to have c :: Color c = undefined convert :: Data a => Int -> a convert i x = let c = dataTypeConstrs (dataTypeOf x) !! (i-1) in fromConstr c and then use it as in convert 1 `asTypeOf` c You'll find out that in most cases the (pseudo) "type annotation" isn't really needed and the type of the value to produce can be determined automatically by the context. Cheers, Stefan
Henry, Ah, pressed send way to early. Of course, the definition should change a little as well: convert :: Data a => Int -> a convert i = x where x = fromConstr ( dataTypeConstrs (dataTypeOf x) !! (i - 1) ) Cheers, Stefan
On Jun 8, 2009, at 7:10 PM, Henry Laxen wrote:
convert :: (Data a, Data b) =Int -a -b convert i x = let c = dataTypeConstrs (dataTypeOf x) !! (i-1) in fromConstr c
I would like to be able to say: x = convert 1 c and have it assign Red to x then I would like to say: y = convert 1 s and have it assign Small to y, however, when I try that I get:
Ambiguous type variable `b' in the constraint: `Data b' arising from a use of `convert' at <interactive>:1:8-18 Probable fix: add a type signature that fixes these type variable(s)
Of course if I say x :: Color = convert 1 c, it works, but I would like to avoid that if possible, as all of the information is already contained in the parameter c. Is there any way to do this? Thanks in advance for your wise counsel.
Best wishes, Henry Laxen
The type signature for 'convert' is throwing away the information you want. Try it with the following type signature and it should work fine: convert :: (Data a) => Int -> a -> a Of course, as has been noted, SYB is a rather big sledgehammer for the insect in question. Cheers, S.
Sterling Clover
Try it with the following type signature and it should work fine:
convert :: (Data a) => Int -> a -> a
Of course, as has been noted, SYB is a rather big sledgehammer for the insect in question.
Cheers, S.
Thank you Sterling. That is the answer I was looking for. I was trying out things from the various Scrap your Boilerplate papers, and got stuck doing something that I thought should be easy. Now it makes sense. Thanks again. Best wishes, Henry Laxen
participants (4)
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Henry Laxen -
Jason Dagit -
Stefan Holdermans -
Sterling Clover