4 ms·

One possible solution is to take a "beautiful fold" http://hackage.haskell.org/package/foldl-1.3.7/docs/Control-Foldl-ByteString.html#v:length http://hackage.ha

One possible solution is to take a "beautiful fold" http://hackage.haskell.org/package/foldl-1.3.7/docs/Control-Foldl-ByteString.html#v:length http://hackage.haskell.org/package/foldl-1.3.7/docs/Control-... and connect it to an effectful stream of bytes http://hackage.haskell.org/package/pipes-bytestring-2.1.6/docs/Pipes-ByteString.html#v:fromHandle http://hackage.haskell.org/package/pipes-bytestring-2.1.6/do... inside the callback of "withFile". Not unlike the Streams/Collectors framework in Java.

taeric 9y ago
But... does that work? Doesn't that leave you open to having something evaluate the end of the "withFile" before you evaluated something in the callback? That is, the only reason what you said is accurate, is because you said "effectful stream of bytes" meaning "eagerly effectful." Right? If you do all of that lazily, you can just as easily shoot yourself in the foot here. (Please nobody take this as a damning criticism of Haskell. I'm not intending it that way.)
- danidiaz 9y ago
  The streams/sinks work in the IO monad and that induces an ordering of the effects, yes. Within the callback, you can still do things like filtering or taking only a part of the stream. Returning the stream itself from the "withFile" and trying to run it later still is still a bug. I like this approach better that conventional Haskell lazy I/O, which I personally find quite confusing.
  - taeric 9y ago
    I guess it just surprises me that having to worry about the state of a stream is a thing I have to do. More, it sounds like I can't really use the typesystem to help here. At least, not from the sound of it.
    - Tarean 9y ago
      In praxis a streaming library like pipes, streaming or conduit handles the file handles for you so application code is safe. Safe-ish in the case of streaming but that is a conscious tradeoff between convenience and safety. You could argue that libraries shouldn't contain unsafe code either and go with a substructural type system, use indexed monads, fake regions or something similar. Whether the extra complexity is worth it is arguable, though.

taeric 9y ago

But... does that work? Doesn't that leave you open to having something evaluate the end of the "withFile" before you evaluated something in the callback? That is, the only reason what you said is accurate, is because you said "effectful stream of bytes" meaning "eagerly effectful." Right? If you do all of that lazily, you can just as easily shoot yourself in the foot here. (Please nobody take this as a damning criticism of Haskell. I'm not intending it that way.)

danidiaz 9y ago
The streams/sinks work in the IO monad and that induces an ordering of the effects, yes. Within the callback, you can still do things like filtering or taking only a part of the stream. Returning the stream itself from the "withFile" and trying to run it later still is still a bug. I like this approach better that conventional Haskell lazy I/O, which I personally find quite confusing.
- taeric 9y ago
  I guess it just surprises me that having to worry about the state of a stream is a thing I have to do. More, it sounds like I can't really use the typesystem to help here. At least, not from the sound of it.
  - Tarean 9y ago
    In praxis a streaming library like pipes, streaming or conduit handles the file handles for you so application code is safe. Safe-ish in the case of streaming but that is a conscious tradeoff between convenience and safety. You could argue that libraries shouldn't contain unsafe code either and go with a substructural type system, use indexed monads, fake regions or something similar. Whether the extra complexity is worth it is arguable, though.