GHC.Internal.Unsafe.Coerce

ghc-internal-9.1001.0: Basic libraries

Safe Haskell | Unsafe
Language | Haskell2010

GHC.Internal.Unsafe.Coerce

Synopsis

- unsafeCoerce :: a -> b
- unsafeCoerceUnlifted :: forall (a :: UnliftedType) (b :: UnliftedType). a -> b
- unsafeCoerceAddr :: forall (a :: TYPE 'AddrRep) (b :: TYPE 'AddrRep). a -> b
- unsafeEqualityProof :: forall {k} (a :: k) (b :: k). UnsafeEquality a b
- data UnsafeEquality (a :: k) (b :: k) where
  - UnsafeRefl :: forall {k} (a :: k). UnsafeEquality a a
- unsafeCoerce# :: a -> b

# Documentation

unsafeCoerce :: a -> b Source #

`unsafeCoerce` coerces a value from one type to another, bypassing the type-checker.

There are several legitimate ways to use `unsafeCoerce`:

1. To coerce a lifted type such as `Int` to `Any`, put it in a list of `Any`, 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`.

1. (superseded) The "reflection trick", which takes advantage of the fact that in `class C a where { op :: ty }`, we can safely coerce between `C a` and `ty` (which have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. It is now preferred to use `withDict` in `GHC.Magic.Dict`, which is type-safe. See Note [withDict] in GHC.Tc.Instance.Class for details.
2. (superseded) Casting between two types which have exactly the same structure: between a newtype of T and T, or between types which differ only in "phantom" type parameters. It is now preferred to use `coerce` from `Data.Coerce`, which is type-safe.

Other uses of `unsafeCoerce` are undefined. In particular, you should not use `unsafeCoerce` to cast a T to an algebraic data type D, unless T is also an algebraic data type. For example, do not cast ``Int`->`Int`` to `Bool`, even if you later cast that `Bool` back to ``Int`->`Int`` before applying it. The reasons have to do with GHC's internal representation details (for the cognoscenti, data values can be entered but function closures cannot). If you want a safe type to cast things to, use `Any`, which is not an algebraic data type.

unsafeCoerceUnlifted :: forall (a :: UnliftedType) (b :: UnliftedType). a -> b Source #

unsafeCoerceAddr :: forall (a :: TYPE 'AddrRep) (b :: TYPE 'AddrRep). a -> b Source #

unsafeEqualityProof :: forall {k} (a :: k) (b :: k). UnsafeEquality a b Source #

data UnsafeEquality (a :: k) (b :: k) where Source #

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.

Constructors

UnsafeRefl :: forall {k} (a :: k). UnsafeEquality a a |

unsafeCoerce# :: a -> b Source #

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.