[Haskell-cafe] Takusen and strictness
bos at serpentine.com
Fri Mar 2 13:30:41 EST 2007
Paul Moore wrote:
> ... ie, there's deep dark magic involved in the seemingly simple
> getContents, which isn't easily available to mere mortals (or even
> semi-immortal library designers).
That's really not true. getContents looks simple from the outside, and
it *can* be simple underneath, too.
You can write a getContents on an arbitrary source of data with just one
built-in action and two other primitives. The built-in is
unsafeInterleaveIO, and the primitives from your data source are "get me
the next item" and "am I done yet?".
getContents :: MyDataItem a => DataSource a -> IO [a]
getContents myDataSource = unsafeInterleaveIO $ do
empty <- amIDoneYet myDataSource
then return 
else do x <- getMeTheNextItem myDataSource
xs <- getContents myDataSource
If you're providing this in a library, you have to consider what you
should do for your consumer if amIDoneYet or getMeTheNextItem does "the
wrong thing", but that's hardly unusual.
You can make the implementation complex, so that it prefetches data,
handles exceptions, and whatnot, but the basic idea isn't too terribly
scary. You'll see this pattern in a number of Haskell libraries
(System.IO, Data.ByteString.Lazy, Control.Concurrent.Chan, and more).
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