How to read bytes from a file in a type-safe AND fast way?

[jerbaroo]

Use of Streamly.FileSystem.File.readFile will not result in any compiler warning/error, even though it is an operation with the potential for errors (e.g. a filenotfound will result in an exception).

I currently have a wrapper over readFile which will catch (using Stream.handle) an IOException and then throw an effectful Error which MUST be handled by the caller (otherwise compilation will fail). HOWEVER this solution inhibits stream fusion :(

Is it possible to have a variant of readFile that has some explicit error returning (perhaps an Either)?

tl;dr: I am looking for the best of both worlds, how to have a version of readFile with 1. errors that MUST be handled by the caller and 2. still has good performance.

[adithyaov]

Use Stream.bracket for resource management.
If you want to introduce a new API, what do you think the signature should look like?

[jerbaroo]

Firstly thanks to you and Harendra for all the work on this library, a very valuable addition to the Haskell ecosystem 🙏🏻

On the issue: it seems like Stream.bracket inhibits stream fusion (therefore not fast) and also "The exception is not caught, it is rethrown." (therefore not enforcing that the caller must handle the error).

The current type signature of read is:

read :: (MonadIO m, MonadCatch m) => FilePath -> Stream m Word8

The following (or similar) would be a great addition, as it forces the caller to acknowledge how to deal with possible errors:

readSafe :: (MonadIO m, MonadCatch m) => FilePath -> (IOException -> m a) -> Stream m Word8

[adithyaov]

Firstly thanks to ...

Appreciate the kind words! Glad you're finding streamly useful.

therefore not fast

Is it not possible to validate your FilePath before passing it to read? If
error handlers are your slow path (path you don't take very often), you can move
error handling to the end, after the stream is consumed.

Error handling is never efficient. But that is a tradeoff you're willing to
make for correctness. In your case, you don't need to use the stream error
handlers. The stream error handlers are particularly useful if the handlers
are of the form (IOException -> Stream m a)