Documentation

Coerce a value from one type to another, bypassing the type-checker.

There are several legitimate ways to use unsafeCoerce:

1. To coerce e.g. Int to HValue, put it in a list of HValue, and then later coerce it back to Int before using it.
2. To produce e.g. (a+b) :~: (b+a) from unsafeCoerce Refl. Here the two sides really are the same type -- so nothing unsafe is happening -- but GHC is not clever enough to see it.
3. In Data.Typeable we have

       eqTypeRep :: forall k1 k2 (a :: k1) (b :: k2).
                    TypeRep a -> TypeRep b -> Maybe (a :~~: b)
       eqTypeRep a b
         | sameTypeRep a b = Just (unsafeCoerce HRefl)
         | otherwise       = Nothing
     

Here again, the unsafeCoerce HRefl is safe, because the two types really are the same -- but the proof of that relies on the complex, trusted implementation of Typeable.

4. The "reflection trick", which takes advantage of the fact that in class C a where { op :: ty }, we can safely coerce between C a and ty (which have different kinds!) because it's really just a newtype. Note: there is no guarantee, at all that this behavior will be supported into perpetuity.

For safe zero-cost coercions you can instead use the coerce function from Data.Coerce.

This type is treated magically within GHC. Any pattern match of the form case unsafeEqualityProof of UnsafeRefl -> body gets transformed just into body. This is ill-typed, but the transformation takes place after type-checking is complete. It is used to implement unsafeCoerce. You probably don't want to use UnsafeRefl in an expression, but you might conceivably want to pattern-match on it. Use unsafeEqualityProof to create one of these.

Highly, terribly dangerous coercion from one representation type to another. Misuse of this function can invite the garbage collector to trounce upon your data and then laugh in your face. You don't want this function. Really.