haskell - Getting the length of a Data.ByteString.Builder -

haskell - Getting the length of a Data.ByteString.Builder -

i have function tabulate takes list of objects, , list of functions turn fields of these objects builders. returns builder representing nicely formatted table. e.g.:

tabulate :: [a -> builder] -> [a] -> builder tabulate = ... info assc = assc { key :: string, value :: string } > allow funcs = [string7 . key, const $ char7 '=', string7 . value ] > allow objs = ["short" `assc` "a", "longer" `assc` "b", "waylongername" `assc` "c"] > hputbuilder stdout $ tabulate funcs objs short = longer = b waylongername = c

to need determine maximum length each column. @ moment using tolazybytestring on each element (which slow).

is possible length of builder without first turning bytestring?

alternatively, there way (efficiently) implement tabulate (with or without using builder)?

looking source of builder, it's defined as

newtype builder = builder (forall r. buildstep r -> buildstep r)

so sequencing builder composing functions, , there no way how length of output without evaluating stack of functions. create own helper module info type combine builders length calculations:

newtype builderl = builderl { bllenght :: !int, blbuilder :: builder } instance monoid builderl mempty = builderl 0 mempty mappend (builderl l1 t1) (builderl l2 t2) = builderl (l1 + l2) (t1 <> t2)

and create helper functions constructing builderls, like

bytestring :: bytestring -> builderl bytestring t = builderl (length t) (bytestring t)

etc. utilize module , builderl tables , you'll have length @ hand.

haskell