On Fri, Jan 16, 2009 at 4:42 PM, eyal.lotem@gmail.com
Hi,
I would like to suggest a change to Haskell prime's handling of imports.
First, let me define a few terms:
Qualified import: import qualified Data.Map [as Map]
Closed-unqualified import: import Data.Map(Map, lookup)
Open-unqualified import: import Data.Map
I believe that the last type, open-unqualified is a really bad idea, and should be discouraged or even disallowed. It should definitely not be the default, like it is today.
These are my reasons:
A) Most importantly, reading code that has even a few open unqualified imports makes deciphering where names are coming from really difficult. The other two forms make it really easy. Code is read more often than it is written -- lets optimize for the reading case.
B) Code that has open unqualified imports may break when new symbols are added to the libraries its importing. This means that libraries cannot even be considered backwards compatible if they retain semantics, but add new exported names.
C) In my experience, Haskell modules can map their semantics to standard type-classes such as Monoid, Functor, Applicative, etc. This means that many Haskell modules can do without exporting many names. Sometimes, they can export no names at all (Only instances). This means that closed-unqualified imports often do just fine. When there are many exported names, qualified imports are good.
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I think currently many modules are designed to be imported unqualified, and this is unfortunate. Haskell' libraries can fix this. For example, the various Monadic counterparts such as mapM, replicateM, etc could do without the "M" in their names, and one could use:
import qualified Control.Monad as M
M.for M.map
and so on.
Additionally, I think there are some strong sentiments against the choice of the function-composition dot as the qualified module name lookup symbol. It discourages people from using qualified imports. Some other symbol ought to be used, but its not clear which symbol is free. Perhaps if unicode operators are allowed, then function composition can move to the "right" symbol.
I believe qualified imports should probably have this syntax:
import Control.Monad as M
Closed-qualified imports can remain in the same syntax. Open-qualified imports should probably be discouraged with a syntax like:
Correction: Closed-unqualified imports can remain in the same syntax. Open-unqualified imports should probably be discouraged with a syntax like:
import unqualified Control.Monad
And symbol clashes with these should probably just always choose the overriding name.
Additionally, it would be nice if open unqualified imported were
imported -> imports
considered quick&dirty enough for -Wall to warn that they generate name clashes or are more difficult to read, to further discourage them.
Eyal