New-to-Haskell, i.e. elementary, Haskell questions; extended discussions

oleg | 19 Oct 2012 08:39

A yet another question about subtyping and heterogeneous collections


First of all, MigMit has probably suggested the parameterization of
Like by the constraint, something like the following:

data Like ctx = forall a. (ctx a, Typeable a) => Like a

instance ALike (Like ALike) where
   toA (Like x) = toA x

instance CLike (Like CLike) where
   toC (Like x) = toC x

get_mono :: Typeable b => [Like ALike] -> [b]
get_mono = catMaybes . map ((\(Like x) -> cast x))

lst_a :: [Like ALike]
lst_a = [Like a1, Like b1, Like c1, Like d1]

lst_c :: [Like CLike]
lst_c = [Like c1, Like d1]

t1 = map print_a lst_a
t2 = map print_a lst_c

(The rest of the code is the same as in your first message). 
You need the flag ConstraintKinds. 

Second, all your examples so far used structural subtyping (objects
with the same fields have the same type) rather than nominal
subtyping of C++ (distinct classes have distinct types even if they

(Continue reading)

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Dmitry Vyal | 19 Oct 2012 20:43

Re: A yet another question about subtyping and heterogeneous collections


> Second, all your examples so far used structural subtyping (objects
> with the same fields have the same type) rather than nominal
> subtyping of C++ (distinct classes have distinct types even if they
> have the same fields; the subtyping must be declared in the class
> declaration). For the structural subtyping, upcasts and downcasts can
> be done mostly automatically. See the OOHaskell paper or the code
Hello Oleg,
I've glanced over both HList and OOHaskell papers when I considered 
taking different approaches. Albeit elegant, OOHaskell looked too heavy 
for my purposes, I don't need mutability, for example. And HList paper 
left me with two questions. The first one is how much such an encoding 
costs both in terms of speed and space. And the second one is can I 
conveniently define a Storable instance for hlists. As I said before, I 
need all this machinery to parse a great number of serialized nested C 
structs from a file.

Best regards
Dmitry

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oleg | 23 Oct 2012 10:14

Re: A yet another question about subtyping and heterogeneous collections


> And HList paper left me with two questions. The first one is how much
> such an encoding costs both in terms of speed and space. And the
> second one is can I conveniently define a Storable instance for
> hlists. As I said before, I need all this machinery to parse a great
> number of serialized nested C structs from a file.

I'm afraid I've overlooked the part about the great serialized C
structs. Serializing HList is easy -- it's de-serialization that is
difficult. Essentially, we need to write a
mini-type-checker. Sometimes, Template Haskell can help, and we can
use GHC's own type-checker.

Since the approach you outlined relies on Haskell type-classes to
express hierarchies, you'll have the same type-checking
problem. You'll have to somehow deduce those type-class constraints
during the de-serialization, and convince GHC of them. If you assume
a fixed number of classes (C struct types), things become simpler. The
HList-based solution becomes just as simple if you assume a fixed
number of record types.