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Diffstat (limited to 'compiler/GHC/Builtin/primops.txt.pp')
| -rw-r--r-- | compiler/GHC/Builtin/primops.txt.pp | 44 |
1 files changed, 0 insertions, 44 deletions
diff --git a/compiler/GHC/Builtin/primops.txt.pp b/compiler/GHC/Builtin/primops.txt.pp index e4283b2dda..e99e5dc3a3 100644 --- a/compiler/GHC/Builtin/primops.txt.pp +++ b/compiler/GHC/Builtin/primops.txt.pp @@ -3845,50 +3845,6 @@ pseudoop "seq" -- This fixity is only the one picked up by Haddock. If you -- change this, do update 'ghcPrimIface' in 'GHC.Iface.Load'. -pseudoop "unsafeCoerce#" - o -> p - { 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 well typed. - - 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 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. - - } - with can_fail = True - --- NB. It is tempting to think that casting a value to a type that it doesn't have is safe --- as long as you don't "do anything" with the value in its cast form, such as seq on it. This --- isn't the case: the compiler can insert seqs itself, and if these happen at the wrong type, --- Bad Things Might Happen. See bug #1616: in this case we cast a function of type (a,b) -> (a,b) --- to () -> () and back again. The strictness analyser saw that the function was strict, but --- the wrapper had type () -> (), and hence the wrapper de-constructed the (), the worker re-constructed --- a new (), with the result that the code ended up with "case () of (a,b) -> ...". - primop TraceEventOp "traceEvent#" GenPrimOp Addr# -> State# s -> State# s { Emits an event via the RTS tracing framework. The contents |