Right, `toLazy` (and thus `toLazy_`, which just drops the final return value) return a strictly evaluated lazy bytestring. They are like `Pipes.ByteString` `toLazyM` and `Pipes.Prelude.toListM` and conduit things like `sinkList` and `sinkLazy`[^1] -- they all just make a "difference list" of each chunk or element, and then when the stream is done, compose them and apply to LBS.empty to make a lazy bytestring / lazy text / list . So you can't inspect the beginning without until you the whole materialized stream in memory. (They don't have to write a new bytestring at least, they just recycle the references to the chunks.) The object of the game is to keep all that from happening, of course, though sometimes it is necessary. I'm not sure I'm following the round-tripping through conduit in the snippets, are you working with a conduit source somewhere?
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