https://github.com/haskell/bytestring/blob/39ad965d17ce8253e1db5343df4ea5836d0edf1d/Data/ByteString/Builder/Prim/Internal.hs#L22-L26 There actually is such a module in the testsuite: https://github.com/haskell/bytestring/blob/39ad965d17ce8253e1db5343df4ea5836d0edf1d/tests/builder/Data/ByteString/Builder/Prim/Tests.hs Is it the one that was meant in the haddocks? I wonder whether the mentioned utilities are actually these: https://github.com/haskell/bytestring/blob/39ad965d17ce8253e1db5343df4ea5836d0edf1d/tests/builder/Data/ByteString/Builder/Prim/TestUtils.hs