haskell - Is there any connection between `a :~: b` and `(a :== b) :~: True`?

Question

Is there any connection implemented between propositional and promoted equality?

Let's say I have

prf :: x :~: y

in scope for some Symbols; by pattern matching on it being Refl, I can transform that into

prf' :: (x :== y) :~: True

like this:

fromProp :: (KnownSymbol x, KnownSymbol y) => x :~: y -> (x :== y) :~: True
fromProp Refl = Refl

But what about the other direction? If I try

toProp :: (KnownSymbol x, KnownSymbol y) => (x :== y) :~: True -> x :~: y
toProp Refl = Refl

then all I get is

? Could not deduce: x ~ y
  from the context: 'True ~ (x :== y)

Answer 1

Yes, going between the two representations is possible (assuming the implementation of :== is correct), but it requires computation.

The information you need is not present in the Boolean itself (it's been erased to a single bit); you have to recover it. This involves interrogating the two participants of the original Boolean equality test (which means you have to keep them around at runtime), and using your knowledge of the result to eliminate the impossible cases. It's rather tedious to re-perform a computation for which you already know the answer!

Working in Agda, and using naturals instead of strings (because they're simpler):

open import Data.Nat
open import Relation.Binary.PropositionalEquality
open import Data.Bool

_==_ : ? -> ? -> Bool
zero == zero = true
suc n == suc m = n == m
_ == _ = false

==-refl : forall n -> (n == n) ≡ true
==-refl zero = refl
==-refl (suc n) = ==-refl n


fromProp : forall {n m} -> n ≡ m -> (n == m) ≡ true
fromProp {n} refl = ==-refl n

-- we have ways of making you talk
toProp : forall {n m} -> (n == m) ≡ true -> n ≡ m
toProp {zero} {zero} refl = refl
toProp {zero} {suc m} ()
toProp {suc n} {zero} ()
toProp {suc n} {suc m} p = cong suc (toProp {n}{m} p)

In principle I think you could make this work in Haskell using singletons, but why bother? Don't use Booleans!