[Haskell-cafe] Design your modules for qualified import
andrewcoppin at btinternet.com
Fri Jun 6 14:35:00 EDT 2008
Johan Tibell wrote:
> * Why is this practice common in Haskell
> Here are some guesses:
> 1. It's common in papers.
> 2. It's the default.
> 3. Lack of common interfaces.
It's really quite frustrating that it is 100% impossible to write a
single function that will process lists, arrays, sets, maps, byte
strings, etc. You have to write several different versions. OK, so some
functions really don't make sense for a set because it's unordered, and
some functions don't make sense for a map, and so forth. But for
example, if I write some complicated algorithm that uses a list to store
data and I change that to a set instead, I now have to wade through the
function changing every operation from a list-op into a set-op. It's
really very annoying!
The problem - as I'm sure everybody is well aware - is that it's rather
hard to come up with a type system formulation that works even though
lists and arrays can store everything, unboxed arrays can only store
types X, Y and Z, sets and maps require ordered data, byte strings only
store bytes, and so on and so forth.
> 4. Haskell is a very expressive language. You can often write a whole
> function definition on one line! Adding those module qualifications
> makes your code slightly longer and it might just break your beautiful
> one liner into two lines.
5. Writing "Hello" Prelude.++ "World" is just ugly. ;-)
In addition, I feel I should point out that only *very* recently did I
discover that it is in fact possible to import a module qualified and
still be able to refer to it easily. What do I mean by that? Well,
suppose I do
I do *not* want to have to write "Text.ParserCombinators.Parsec.runParser"!!
Until very recently, it was not at all clear to me that there is
actually a very simple solution to this problem:
import Text.ParserCombinators.Parsec as P
Now I only have to write "P.runPaser", which is much shorter.
This fact probably needs to be mentioned more loudly - I'm sure I'm not
the only person to have overlooked it...
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