The function unsafeCoerce#
allows you to side-step the
typechecker entirely. That is, it allows you to coerce any type into
any other type. If you use this function, you had better get it right,
otherwise segmentation faults await. It is generally used when you
want to write a program that you know is well-typed, but where
Haskell's type system is not expressive enough to prove that it is
The following uses of unsafeCoerce#
are supposed to work
(i.e. not lead to spurious compile-time or run-time crashes):
- Casting any lifted type to Any
- Casting Any back to the real type
- Casting an unboxed type to another unboxed type of the same size.
(Casting between floating-point and integral types does not work. See
the GHC.Float module for functions to do work.)
- Casting between two types that have the same runtime
representation. One case is when the two types differ only in
"phantom" type parameters, for example Ptr Int to Ptr
Float, or [Int] to [Float] when the list is
known to be empty. Also, a newtype of a type T has
the same representation at runtime as T.
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
, even if you later cast
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.
this can fail with an unchecked exception.