All, So here's the Put monad for the binary serialisation stuff: newtype Put a = Put { runPut :: (a -> Buffer -> [B.ByteString]) -> Buffer -> [B.ByteString] } data Buffer = Buffer {-# UNPACK #-} !(ForeignPtr Word8) {-# UNPACK #-} !Int -- offset {-# UNPACK #-} !Int -- used bytes {-# UNPACK #-} !Int -- length left This is all good, and pretty quick. Code like the following gets turned into great low level code: foo :: Word8 -> Put () foo !n = do word8 n word8 (n+1) word8 (n+17) It does a straight line sequence of writes into memory (there are rules that combine the bounds checking of adjacent writes). After that it calls the continuation with a new buffer. While almost everything is unboxed, the parameters to the continuation are not unboxed. So we have to allocate a new Buffer structure and pass that to the continuation: let { sat_s1F8 = NO_CCS PutMonad.Buffer! [ww1_s1Es ww2_s1Et ww3_s1Ew sat_s1F4 sat_s1F6]; } in w_s1DY GHC.Base.() sat_s1F8; w_s1DY being the continuation here. However we know that really the parameters to this continuation could really be unboxed since we construct all these continuations explicitly in this module. We could re-write the monad type by manually explicitly unboxing the Buffer argument: newtype Put a = Put { runPut :: (a -> Addr# -> ForeignPtrContents -> Int# -> Int# -> Int# -> [B.ByteString]) -> Addr# -> ForeignPtrContents -> Int# -> Int# -> Int# -> [B.ByteString] } Then we'd get no allocations and no heap checks in the fast path. Note that we could still get a continuation in from the 'outside' with the original type and convert it to one with the above type, though obviously that involves unpacking the arguments. This unpacking is basically what the wrapper of a worker/wrapper pair does of course. Obviously this is ugly. We'd much rather write the original and with just a couple annotations get the above representation. This is what I would like to write: newtype Put a = Put { runPut :: (a -> {-# UNPACK #-} !Buffer -> [B.ByteString]) -> {-# UNPACK #-} !Buffer -> [B.ByteString] } So I'm declaring a type and a data constructor that contains a function that is strict in one of it's arguments. I do not wish to distinguish the strictness in the type however, that is it should be perfectly ok to do this: foo :: Foo -> Buffer -> [B.ByteString] Put foo newBuffer :: Put Foo Suppose that in this case we do not know locally that foo is strict and can take it's args unboxed then upon applying the Put constructor we just have to apply a wrapper function that boxes up the args and supplies them to the wrapped function. Obviously that'd be a pessimisation, having to re-box args, however we can expect programmers to only use that UNPACK pragma when they know that it is going to be a win overall. So the ! on the arg is a semantic change and the pragma is a representation change but not a semantic change. The ! means the obvious thing, that the function is strict in that arg. So either the caller or calle must ensure the arg is evaluated to WHNF. Hmm, is the UNPACK actually needed then? Normally when ghc determines that a func is strict in an arg it make the worker unbox that arg if possible and that always seems to be a win. Mine you, in that case we know what the function is going to do with each arg, where as here we do not so perhaps a separate UNPACK makes sense. This issues crops up quite a bit with Monads I think. For example GHC defines it's IO monad to return an unboxed tuple: newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #)) This is fine for low level internal code, but for everyday monads it'd be nice to be able to write nice code and still get great performance. This applies to monads particularly since they very often fully encapsulate the function used as the internal representation, that is all the sites where the representation function is constructed and taken apart are in a single module. So we can 'see' that all uses are strict in some arg. Or if they're not we might want them to be. So instead of doing an analysis to figure out if we're always using it strictly (which is something that jhc/grin might be able to do?) we can just declare the data representation to be strict like we do with ordinary algebraic data types, that way it propagates strictness to the usage sites which is much more convenient than going around strictifying all the usage sites in an attempt to make an analysis notice that it's safe to do a representation change. Ok, I'm done. :-) Duncan