add toStrictByteString

[fumieval]

This change saves the labour of having to import Data.ByteString.Lazy.toStrict just to obtain a strict ByteString.

[Bodigrim]

This change saves the labour of having to import Data.ByteString.Lazy.toStrict just to obtain a strict ByteString.

toStrict is available from Data.ByteString as well, which is presumably already imported if you intend to work with strict bytestrings.

[fumieval]

Oh, I hadn't realised that it is available since 0.11!

Still, I think it's a bit more convenient to have a strict counterpart in Data.ByteString.Builder.

[Bodigrim]

I'm somewhat reluctant to add a trivial composition of toLazyByteString and toStrict under its own name; see haskell/text#295 (comment) for related discussion. @clyring what do you think?

[clyring]

I'm mildly in favor as far as API design goes.

L.toStrict . toLazyByteString is not the only reasonable implementation. (Accumulating chunks in reverse order and fixing that up at concatenation time would even save allocating a few thunks. But I'm not sure that's enough of an improvement to be worth providing.)

[Bodigrim]

The thing is that if you want to build a StrictByteString you'd be better off using strict builders such as bytestring-strict-builder or text-builder-linear.

What happens in practice way too often is that a user does not really care about performance at all and just needs itoa :: Int -> StrictByteString, and the only way to implement it is Data.ByteString.toStrict . Data.ByteString.Builder.toLazyByteString . Data.ByteString.Builder.intDec. This is mouthful, I agree.