[Haskell-beginners] Conciseness question

[Manfred Lotz]

On Sun, 7 Aug 2011 18:55:37 +0200
Ertugrul Soeylemez <es at ertes.de> wrote:

Thanks a lot for your suggestions. 

> 
> This is possible in Haskell using the mapM_ function, but it doesn't
> really solve your problem.  What you need is a foldable data
> structure, which gives its entries names.  As suggested earlier,
> Data.Map gives you such a structure.  You can define a custom index
> type like this:
> 
>     data AppDir
>         = ConfigAppDir
>         | DataAppDir
>         | OtherAppDir
>         deriving Ord
> 
> Then you can create a map from AppDir to a string:
> 
>     import Data.Map (Map)
> 
>     type AppDirs = Map AppDir FilePath
> 
> This is a foldable data structure.  Instead of mapM_ from the Prelude
> you can now use mapM_ or forM_ from Data.Foldable.  To access
> individual directories you can either use safe lookups or unsafe
> lookups:
> 
>     import Data.Map (Map, (!), lookup)
> 
>     lookup ConfigAppDir myDirs
>     myDirs ! ConfigAppDir
> 

This is the solution I like. I have to accept that here I cannot reach
the conciseness of which might be due to Haskell being strongly typed.


> The former is the safe variant giving you a Maybe String, while the
> latter is the unsafe variant, which throws an exception, if the
> directory in question is not present.
> 

Is the latter one really unsafe? I'm not quite sure how to code
something that the compiler accepts and crashes at runtime because 
mydirs :: Map AppDir FilePath and I would believe that the compiler
would detect if the values after the ! is not from AppDir.



-- 
Manfred