Instead of passing 'undefined :: a', I think the current best practice is to pass 'Proxy :: Proxy a' (or in fact 'proxy a', i.e. be polymorphic in the proxy). This way you can never accidentally use the undefined value. Erik On Sun, Apr 13, 2014 at 4:15 PM, J. Stutterheim wrote:

Short, simple, thanks! Works like a charm.

- Jurriën

On 13 Apr 2014, at 15:36, Sjoerd Visscher wrote:

...

Make the match on (x :*: y) irrefutable:

gprintDT ~(x :*: y) = ...

Sjoerd

On 13 Apr 2014, at 15:22, J. Stutterheim wrote:

...

Hi all,

I'm trying to use GHC.Generics to print a data type's structure. I have a test type TestRecord:

data TestRecord = TestRecord { trId :: Int , someStr :: String } deriving Generic

And a PrintDT class:

class PrintDT a where printDT :: a -> String

default printDT :: (Generic a, GPrintDT (Rep a)) => a -> String printDT = gprintDT . from

With a corresponding instance for TestRecord. The GPrintDT class and instances are defined as follows:

class GPrintDT f where gprintDT :: f a -> String

instance (GPrintDT a, GPrintDT b) => GPrintDT (a :*: b) where gprintDT (x :*: y) = " { " ++ gprintDT x ++ ", " ++ gprintDT y ++ " }"

instance (Datatype d, GPrintDT f) => GPrintDT (D1 d f) where gprintDT d = "data " ++ datatypeName d ++ " = " ++ (gprintDT $ unM1 d)

instance (Constructor c, GPrintDT f) => GPrintDT (C1 c f) where gprintDT con | conIsRecord con = conName con ++ (gprintDT $ unM1 con) | otherwise = "No record"

instance (Selector s, GPrintDT a) => GPrintDT (S1 s a) where gprintDT m = selName m

instance (PrintDT a) => GPrintDT (K1 i a) where gprintDT _ = ""

instance PrintDT Int where printDT n = show n

instance PrintDT String where printDT xs = xs

In a first attempt, I apply printDT to a TestRecord value:

test1 :: String test1 = printDT (TestRecord 1 "foo")

This prints the expected result:

"data TestRecord = TestRecord { trId, someStr }"

Now ideally, I wouldn't have to specify some value of TestRecord to get this output, since so far, I'm only printing the structure of the TestRecord type, not the values. I would want the following to give me the same result string:

test2 :: String test2 = printDT (undefined :: TestRecord)

With the current implementation, test2 gives me the following output:

"data TestRecord = TestRecord*** Exception: Prelude.undefined

Is there a way to implement GPrintDT such that test2 gives me the same output as test1?

Thanks!

- Jurriën _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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