On Friday, September 15, 2017 at 7:17:33 PM UTC+2, Gabriel Gonzalez wrote:
>
> Note that this applies to the type that you gave, too! There is no way to
> deduce from the type that the initial state won't be used to produce a
> value :)
>
Ah, you're right, of course. The "done" function may emit stuff without any
input. The problem is that if I remove it and go the Mealy machine route
some ways of splitting seem to become impossible. For example, imagine
splitting a text stream on a two-char separator (while removing the
separator).
>
> However, looking at this more closely I'm not sure that your original type
> for `delimit` is sufficiently general to handle most use cases for grouping
> a stream. If I'm reading the type correctly I think this assumes that you
> have one group of `b`s per original `a` in the input stream, which might
> not be the case. For example, consider this function that groups a stream
> of text chunks into lines:
>
> lines :: Monad m => Stream (Of Text) m r -> Stream (Stream (Of Text))
> m r
>
> I don't see how you could implement that in terms of `delimit`
>
>
>
Instead of the step function returning *NonEmpty b*, it should return
*NonEmpty
[b]:*
*delimit :: Monad m => (x -> a -> (x, NonEmpty [b])) -> (x -> NonEmpty [b])
-> x -> Stream a m r -> Groups b m r*
I think this covers the case in which we encounter multiple newlines in a
single block of text. It would be like:
lines :: Stream T.Text IO r -> Groups T.Text IO r
lines =
Y.delimit (\nl txt ->
if T.null txt
then (nl,pure [])
else let nl' = T.last txt == '\n'
in case (nl,T.lines txt) of
(True,ls) -> (nl', [] :| map pure ls)
(False,l:ls) -> (nl', [l] :| map pure ls)
(_,[]) -> error "never happens")
(\_ -> pure [])
True
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