Difficulty composing log messages for the RIO.Logger via UTF8Builder

Question

I'm using the RIO monad and RIO libraries and want to add logging. The RIO log functions, like logInfo, take a Utf8Builder as parameter, which is a wrapper around a ByteString builder. I would like to efficiently insert log statements in my code. However, I am not sure how to transform simple Text, String and ByteString values into the appropriate Builder.

This does not type-check:

logInfo $ "Identifier: " <> UUID.toByteString myId

where myId is a UUID value. The type checker error is

    • Couldn't match expected type ‘Utf8Builder’
                  with actual type ‘bytestring-0.10.10.0:Data.ByteString.Lazy.Internal.ByteString’

I tried converting the UUID into Text instead, tried to use encodeUtf8Builder and then wrap the resulting Builder in an Utf8Builder, but to no avail.

My gut feeling says that logging should simply work, without lots of conversions; so I am probably missing the obvious route. How can I simply log values using the RIO logger?

Answer 1

What you want is a Display instance for your UUID, which will allow you combine with Utf8Builder efficiently and easily use it in your logs.

instance Display UUID.UUID where
  display = displayBytesUtf8 . UUID.toASCIIBytes

The only reason why displayBytesUtf8 can be used is because you know the content is in ascii. In other words it is your responsibility when creating Display instance that it produces correctly encoded utf8 bytes.

After that you can use it in your logs like so:

logInfo $ "Identifier: " <> display myId

Creating Display instance isn't strictly speaking necessary, you can just use a helper function if you don't want to create orphan instances or wrapper newtypes. This will do just fine:

displayUUID :: UUID.UUID -> Utf8Builder
displayUUID = displayBytesUtf8 . UUID.toASCIIBytes

Reason why I mention Display is because it is idiomatic approach in RIO for logging with custom types.