[Haskell-cafe] lazily traversing a foreign data structure

[Graham Fawcett]

Hi folks,

I'm writing a Gnu DBM module as an exercise for learning Haskell and
its FFI. I'm wondering how I might write a function that returns the
database keys as a lazy list. I've wrapped the two relevant foreign
functions:

    firstKey :: Ptr Db -> IO (Maybe String)
    nextKey  :: Ptr Db -> String -> IO (Maybe String)

NextKey takes a key, and returns the next one. Either function could
return Nothing, since the db may have 0 or 1 keys.

Given these, is it possible to write a (simple) function

    allKeys :: Ptr Db -> IO [String]

that lazily fetches the keys? (Or, an idiomatic way of achieving the
same end?)

Thanks,

Graham