FileStore Feels OO (not FP)

[thomas hartman]

I had some musings on the FileStore library here:

http://blog.patch-tag.com/2009/03/18/fields-whose-type-is-an-impure-function-are-evil-because-i-say-so/

The main conclusion is that this would seem nicer if it took advantage
of typeclassing somehow so that filestores for darcs, git, etc would
be different types.

[Matthew Elder]

Yay! Go blog posts ;)

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[Matthew Elder]

Yay! Go blog posts ;)

On 3/17/09, thomas hartman <thomash...@googlemail.com> wrote:
>

[John MacFarlane]

+++ thomas hartman [Mar 17 09 19:54 ]:

That's interesting. If you look through the darcs history, you'll
see that at one point we did try using typeclasses. There was a FileStore
type class, and a SearchableFileStore type class, and instances for
git, darcs, and so on. We abandoned this, if I remember correctly,
because it made things much more difficult for the calling program. For
example, Gitit's state looks like this:

data AppState = AppState {
sessions :: Sessions SessionData,
users :: M.Map String User,
config :: Config,
filestore :: FileStore,
mimeMap :: M.Map String String,
cache :: Cache,
template :: T.StringTemplate String,
jsMath :: Bool,
plugins :: [Plugin]
}

filestore is of type FileStore and can be either a git or a darcs
filestore.

, filestore = case repository conf of
Git fs -> gitFileStore fs
Darcs fs -> darcsFileStore fs

But how would this go if FileStore were a type class, not a type?

, filestore = case repository conf of
Git fs -> GitFileStore fs
Darcs fs -> DarcsFileStore fs

But this doesn't typecheck, because GitFileStore fs and
DarcsFileStore fs have different types...

John

[Thomas Hartman]

After playing with this for a while, I think I agree... for now :)

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