Re: [Haskell-cafe] Problems with Type family and Typeable

31 Jul 2017


      Thanks! Could you elaborate a little more about the Typeable constraint?
What argument does it add during runtime? In the beginning I thought this
constraint is redundant, as what it claims is always true in this case. If
my understanding is correct, the purpose of this constraint is to tell GHC
to retain the necessary type annotations in runtime?

On Mon, Jul 31, 2017 at 5:41 AM Li-yao Xia  wrote:
...
Hello,
"Data.Type.Equality.EqStar s t || Elem s ts" resolves to True or False,
which are values of type Bool.
To understand why that constraint can't be solved, think about how such a
function is compiled. At run time, types are erased.
A "Proxy p" value carries no more information than unit "()", and a
"Tagged _ a" value is in fact just an "a" value.
So elemTag is compiled to something equivalent to a function of this type,
which has no way of performing the comparison you requested since the
type-level boolean was erased:
-- Original function
     elemTag :: Proxy s -> Tagged (t ': ts) a -> Bool
-- After type erasure
     elemTag :: () -> a -> Bool
As suggested by the type error, you can reify the boolean "Elem s (t ':
ts)" by adding a Typeable constraint, which gets compiled to an additional
run time argument.
elemTag :: forall a s t ts
             .  Typeable (Elem s (t ': ts))
             => Proxy s
             -> Tagged (t ': ts) a
             -> Bool
Li-yao
On 07/31/2017 07:15 AM, Kai Zhang wrote:
...
I got an error when compiling the following codes:
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE UndecidableInstances #-}
import Data.Type.Bool
import Data.Type.Equality
import Data.Tagged (Tagged(..))
import Data.Typeable
type family Elem x  xs :: Bool where
     Elem _ '[] = 'False
     Elem a (x ': xs) = a == x || Elem a xs
elemTag :: forall a s t ts . Proxy s
         -> Tagged (t ': ts) a -> Bool
elemTag _ _ = if typeOf (Proxy :: Proxy (Elem s (t ': ts))) == typeOf
(Proxy :: Proxy 'True)
                  then True
                  else False
GHC says:
• No instance for (Typeable
                          (Data.Type.Equality.EqStar s t || Elem s ts))
         arising from a use of ‘typeOf’
     • In the first argument of ‘(==)’, namely
         ‘typeOf (Proxy :: Proxy (Elem s (t : ts)))’
       In the expression:
         typeOf (Proxy :: Proxy (Elem s (t : ts)))
         == typeOf (Proxy :: Proxy True)
       In the expression:
         if typeOf (Proxy :: Proxy (Elem s (t : ts)))
            == typeOf (Proxy :: Proxy True) then
             True
         else
             False
My question: why is ghc unable to deduce that "Data.Type.Equality.EqStar
s
t || Elem s ts" resolves to "Bool" which should be an instance of
Typeable?
How to fix this?
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