
Hi Everyone,
I tried to come up with a solution, but it has plenty of drawbacks.
data Tag = Hdf5 | Cbf | Unchecked
data A :: Tag -> Type where
Ca :: Symbol -> A Unchecked
Ch :: A Hdf5
Cb :: A Cbf
type family Typestring (a :: A Unchecked) :: Type
type instance Typestring (Ca "hdf5") = A Hdf5
type instance Typestring (Ca "cbf") = A Cbf
data SUnchecked a where
Sh :: SUnchecked (Ca "hdf5")
Sc :: SUnchecked (Ca "cbf")
This checkValue function is useless, because you need to pass A Unchecked,
and constructor (Ca) takes Symbol rather than String, and value of type
SUnchecked a.
checkValue :: forall (a :: A Unchecked). A Unchecked -> SUnchecked a ->
Typestring a
checkValue (Ca x) Sh = Ch
checkValue (Ca x) Sc = Cb
When I tried to run the above code
*Main> :t checkValue (Ca (someSymbolVal "hdf5"))
<interactive>:1:17: error:
• Couldn't match expected type ‘Symbol’
with actual type ‘GHC.TypeLits.SomeSymbol’
• In the first argument of ‘Ca’, namely ‘(someSymbolVal "hdf5")’
In the first argument of ‘checkValue’, namely
‘(Ca (someSymbolVal "hdf5"))’
In the expression: checkValue (Ca (someSymbolVal "hdf5"))
What I really want is something like this, but the problem is I can't do
pattern matching on symbols, and if I change the data type to String from
Symbol then it won't compile .
Could some one point me how to solve this problem?
checkValue :: forall (a :: A Unchecked). SUnchecked a => A Unchecked ->
Typestring a
checkValue (Ca "hdf5") = Ch
checkValue (Ca "cbf") = Cb
Best regards,
Mukesh
On Thu, Oct 4, 2018 at 11:11 PM Francesco Ariis
Hello Nikita,
On Thu, Oct 04, 2018 at 02:50:09AM +0300, Никита Фуфаев wrote:
As this problem requires type to depend on runtime value, you need singletons.
Many thanks for showing us the way, very elegant solution. _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners