[Haskell-cafe] Re: language proposal: ad-hoc overloading

Brandon S. Allbery KF8NH allbery at ece.cmu.edu
Sun Sep 7 16:56:07 EDT 2008

On 2008 Sep 6, at 19:09, John Smith wrote:
> Ryan Ingram wrote:
>> module Prob where
>> import qualified Data.Map as M
>> ....
>> newtype Prob p a = Prob { runProb :: [(a,p)] }
>> combine :: (Num p, Ord a) => Prob p a -> Prob p a
>> combine m = Prob $
>>    M.assocs $
>>    foldl' (flip $ uncurry $ M.insertWith (+)) M.empty $
>>    runProb m
>> Do you see it?  All those "M." just seem dirty to me, especially
>> because the compiler should be able to deduce them from the types of
>> the arguments.
> May I humbly suggest a much simpler solution to your problem: if an  
> identifier is ambiguous, the compiler will use the last import. So,  
> in your example, the compiler will assume that any instance of empty  
> is Data.Map.empty

I don't like that idea very much; if I reorder my imports the program  
semantics suddenly change?

> Some means of using an imported module as the default namespace, and  
> requiring the Prelude to be qualified, may also help.

You can already do this by importing Prelude explicitly, possibly with  
the NoImplicitPrelude language option.

brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH

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