[[email protected]: Re: [GHC] #1218: Add sortNub and sortNubBy to Data.List]

Jan-Willem Maessen jmaessen at alum.mit.edu
Tue Mar 20 22:16:18 EDT 2007


On Mar 20, 2007, at 6:53 PM, Duncan Coutts wrote:

> On Tue, 2007-03-20 at 14:11 +0100, Nils Anders Danielsson wrote:
>> On Tue, 20 Mar 2007, Duncan Coutts <duncan.coutts at worc.ox.ac.uk>  
>> wrote:
>>
>>> H98 requires that Eq and Ord be equality classes and total orders
>>
>> Does it? I was under the impression that a compiler can never assume
>> that any laws hold of any user-defined instances (to do  
>> optimisations,
>> for instance).
>>
>> The report states the following:
>>
>>   "The Eq class provides equality (==) and inequality (/=) methods."
>>   "The Ord class is used for totally ordered datatypes."
>>
>> I interpret this as being mere usage guidelines, and nothing more.
>
> Aye.

Hmm, I've always taken these to be binding contracts.  For example,  
do I get to assume either == or <= are transitive?  But I admit this  
is at the level of writing libraries---so that, as another poster  
noted, I'm allowed to use a sort function which yields a different  
result (possibly _|_) if I don't obey the contract.

But I'd often like my compiler to assume that <= is transitive...

-Jan-Willem Maessen

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