Newtype wrappers

Edward Kmett ekmett at
Tue Jan 15 00:49:15 CET 2013

No magic coercing is present in the proposal. You need to use explicit newtype wrap and newtype unwrap expressions.

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On Jan 14, 2013, at 6:42 PM, Johan Tibell <johan.tibell at> wrote:

> On Mon, Jan 14, 2013 at 3:40 PM, Evan Laforge <qdunkan at> wrote:
>> Wait, what's the runtime error?  Do you mean messing up Set's invariants?
> Yes.
>> If you as the library writer don't want to allow unsafe things, then
>> don't export the constructor.  Then no one can break your invariants,
>> even with newtype malarky.  If you as the the library user go and
>> explicitly import the bare Set constructor from (theoretical)
>> Data.Set.Unsafe, then you are in the position to break Set's internal
>> invariants anyway, and have already accepted the great power / great
>> responsibility tradeoff.
> If it's explicit that this is what you're doing I'm fine with it. I
> just don't want magic coercing depending on what's in scope.
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