[Haskell-cafe] Streaming a Conduit into a lazy list

Michael Snoyman michael at snoyman.com
Wed Nov 4 19:41:29 UTC 2015

On Wed, Nov 4, 2015 at 9:28 AM, Jules Bean <jules at jellybean.co.uk> wrote:

> On 4 Nov 2015, at 13:04, Michael Snoyman <michael at snoyman.com> wrote:
> This got me curious, so I just added a `sourceToList` function to master:
> https://github.com/snoyberg/conduit/commit/289f671cb7669c4aec78d8e77f01f2ace165d73a
> Having spent a day thinking this over…
> Nothing with the type `Source m a -> m [a]` can work for my second example
> - the one where I use runExceptionT to discharge the MonadThrow constraint.
> This is because once you use runExceptionT you have pushed yourself into
> the situation where in case of error there is no ‘return value’.
> It’s not that I care about that per se - if there is an error then the
> return value is no use to me - but unfortunately that has knock-on
> implications on laziness.
> The Writer monad solution pushes out the return value incrementally by a
> ‘side-channel’ rather than using the return value and it’s that property
> which lets it work even in the presence of runExceptionT.
> Another approach which would work though is to provide a newtyped Identity
> monad which handles MonadThrow by _|_, which would allow you to regain
> laziness (?)
> Jules

I don't think that the Writer example above is demonstrating what you're
saying, since you're using imprecise exceptions (`error`) instead of
MonadThrow. You could do the same thing with sourceToList and get that

You're also correct that some kind of a Identity monad with a MonadThrow
instance based on `throw` would allow this to work.

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