I think this is a feasible idea. Whether this is in fact a good idea is an entirely separate question, however, and I want feedback.
Currently, I believe that an UNPACK pragma used on a multi-constructor type has no effect. For example,
data Node = Node Int {-# UNPACK #-} !(Maybe Node) {-# UNPACK #-} !(Maybe Node)
is the same as
data Node = Node Int !(Maybe Node) !(Maybe Node)
What I'd like is for this to translate into four constructors, one for each combination of constructors for the UNPACK'd fields:
data Node = Node0 Int -- Nothing, Nothing
| Node1 Int Node -- Just, Nothing
| Node2 Int Node -- Nothing, Just
| Node3 Int Node Node -- Just, Just
The primary counterargument I can think of is that this can result in a single-constructor type being turned into a multi-constructor type, which runs the risk of interfering with GHC's sexcellent handling of single-constructor types.
The countercounterargument is, of course, that the {-# UNPACK #-} pragma already behaves differently depending on the single-constructorness of the underlying type, and that the obligation is already on the programmer to check such things.
For reference, could somebody point me to the place in GHC that currently takes care of {-# UNPACK #-} pragmas, so I could -- for instance -- figure out whether or not there's another reason that this idea isn't in place already?
Louis Wasserman
wasserman.louis@gmail.com