proposal: add 'unsafeCoerce'

[kahl at cas.mcmaster.ca]

Simon Marlow <simonmarhaskell at gmail.com> wrote:
 > 
 > >  From Section 4.2.3, from the Haskell report:
 > > 
 > > A declaration of the form
 > > 
 > > newtype cx => T u1 ... uk = N t
 > > 
 > > introduces a new type whose representation is the same as an existing  
 > > type. The type (T u1 ... uk) renames the datatype t. It differs from  a 
 > > type synonym in that it creates a distinct type that must be  explicitly 
 > > coerced to or from the original type. Also, unlike type  synonyms, 
 > > newtype may be used to define recursive types. The  constructor N in an 
 > > expression coerces a value from type t to type (T  u1 ... uk). Using N 
 > > in a pattern coerces a value from type (T u1 ...  uk) to type t. These 
 > > coercions may be implemented without execution  time overhead; newtype 
 > > does not change the underlying representation  of an object.
 > 
 > *blink*
 > 
 > I stand corrected (for the second time today, duh, maybe I should check facts 
 > before trusting my memory next time...).
 > 
 > I have no idea why the report does say that though.  Seems very odd, there's no 
 > need to mention the representation.  Indeed, the language provides no way 
 > (absent unsafeCoerce) for a programmer to determine what the representation is, 
 > so how should we interpret that paragraph?  An invisible requirement or an 
 > implementation hint?

Or as a hint that we should introduce a ``pseudo-polymorphic pseudo-function''

safeCoerce :: a -> b

which can only be used if a and b have the same representation,
as mandated by the language definition?
(Or if the compiler can derive that for the type language
 currently used...)



Wolfram