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Static pointer expressions can not have free variables. For example:

addPointers :: StaticPtr Int -> StaticPtr Int -> StaticPtr Int
addPointers x y = static (deRefStaticPtr x + deRefStaticPtr y)

would be invalid, since x and y are free variables. My proposal would be to make this code sample valid. In particular, my proposal is to allow free variables in static expressions if the free variables of the type StaticPtr a for some type a.

Let's say that m n are Static pointers to integers. The serialized form would of addPointers m n would be:

A pointer to the code deRefStaticPtr x + deRefStaticPtr y

The serialized form of m, associated with x.

The serialized form of n, associated with y.

When this is sent over the wire and dereferenced, the other machine would

Find m and n's pointers

Find the code deRefStaticPtr x + deRefStaticPtr y

Evaluate deRefStaticPtr x + deRefStaticPtr y, substituting m's value for x, and n's value for y.

The reason this feature would be useful is would let the user modify and combine static pointers.

You might ask "why not let the user create a GADT to do that (like in https://hackage.haskell.org/package/distributed-static-0.3.8/docs/Control-Distributed-Static.html\#t:Static)?" The reason is that then it won't fuse (with recreating every fusion rule for that GADT). For example, if m is static 7 and n is static 3, then addPointers m n can fuse to static 11.

This is more important then it seems. For example, say you have composePointers :: StaticPtr (a -> b) -> StaticPtr (b -> c) -> StaticPtr (a -> c), and you compose a bunch of pointers. With the GADTS, every composePointers would be its own node, but with GHC support, it would probably fuse.

In general, dealing with a GADT requires a lot of "glue code" that can be eliminated with GHC support. I'm not sure if this proposal makes a performance difference (it probably makes it more efficient), but it would make code more elegant, cutting out all the glue code.

Trac metadata

Trac field	Value
Version	8.2.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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