proposal: add 'unsafeCoerce'

Robert Dockins robdockins at
Tue Nov 21 09:47:28 EST 2006

On Nov 21, 2006, at 8:01 AM, Simon Marlow wrote:

> Malcolm Wallace wrote:
>> Taral <taralx at> writes:
>>> On 11/20/06, Malcolm Wallace <Malcolm.Wallace at> wrote:
>>>>    newtype Wrapper a = Wrap a
>>>>    convert :: [a] -> [Wrapper a]
>>>>    convert xs = map Wrap xs
>>> Interesting! Looks like the compiler lacks rules for optimizing "map
>>> id" &c.\
>> And not all compilers have optimisation phases.
>>> Your coercion does assume that the underlying runtime doesn't have
>>> some kind of type-tag implementation of type classes.
>> I believe this is guaranteed by the definition of newtype in the
>> Language Report.
> The language doesn't say anything about the runtime representation  
> of newtype. It so happens that the semantics lead to the obvious  
> implementation of a newtype   as a type cast (that was the reason  
> for introducing newtype, after all), but there's nothing to say you  
> have to implement it this way.  It would be wrong to require that  
> unsafeCoerce let you convert between a newtype and its underlying  
> type across implementations.

 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.

I'd say that discusses the runtime representation.

> Cheers,
> 	Simon

Rob Dockins

Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
           -- TMBG

More information about the Libraries mailing list