An import trick too useful to pass up

So here is a trick I learned from reading the Snap code over the last week.

Namespace collisions suck. Data.Map, Data.Set and Data.List all have fairly similar functions that we all know and love to use, and they differ subtly, so people often import them qualified, i.e.

import qualified Data.Map as M

import qualified Data.Set as S

import qualified Data.List as L

Now the annoying thing about this is that then you have to prepend the type signatures too, e.g.

foobar :: S.Set a -> a -> S.Set a

This is pissy, so what some genius who worked on Snap did was:

import Data.Map (Map)

import qualified Data.Map as M

import Data.Either (Left,Right)

Now this sounds simple and all, but it actually works Much Better in practice than in theory, partly because type constructors like Left and Right rarely overlap from module to module.

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