id	summary	reporter	owner	description	type	status	priority	milestone	version	resolution	keywords	cc
60	Add support for additional implementations of fusible functions	rl		"The prime example is `replicate`: if it doesn't fuse, we really want it to use `basicSet` rather than `unstream`. There are many other functions like this. We don't really want to rely on rules for this because that would require an additional simplifier phase to work reliably. Rather, we'd want to implement `replicate` roughly like this:

{{{
unstreamOrElse :: Stream a -> v a -> v a
{-# INLINE_STREAM unstreamOrElse #-}
unstreamOrElse s x = x

{-# RULES

""stream/unstreamOrElse""
  stream (unstreamOrElse s x) = s

  #-}

replicate n x = unstreamOrElse (Stream.replicate n x)
                               (new (Mutable.replicate n x))
}}}

We'll probably also want `streamOrElse`:

{{{
streamOrElse :: (Stream a -> b) -> (v a -> b) -> v a -> b
{-# INLINE_STREAM streamOrElse #-}
streamOrElse f g = g

{-# RULES

""streamOrElse/unstream""
  streamOrElse f g (unstream s) = f s

""streamOrElse/unstreamOrElse""
  streamOrElse f g (unstreamOrElse s x) = f s
}}}

There are ways to keep the number of rules manageable (e.g., by making `streamOrElse` and `unstreamOrElse` the only streaming combinators."	enhancement	new	major	0.10				haskell.org@… v.dijk.bas@…