Technical report Frank / Johann-Wolfgang-Goethe-Universität, Fachbereich Informatik und Mathematik, Institut für Informatik
Program Equivalence for a Concurrent Lambda Calculus with Futures
- Reasoning about the correctness of program transformations requires a notion of program equivalence. We present an observational semantics for the concurrent lambda calculus with futures Lambda(fut), which formalizes the operational semantics of the programming language Alice ML. We show that natural program optimizations, as well as partial evaluation with respect to deterministic rules, are correct for Lambda(fut). This relies on a number of fundamental properties that we establish for our observational semantics.
On conservativity of concurrent Haskell
- The calculus CHF models Concurrent Haskell extended by
concurrent, implicit futures. It is a process calculus with concurrent threads, monadic concurrent evaluation, and includes a pure functional
lambda-calculus which comprises data constructors, case-expressions,
letrec-expressions, and Haskell’s seq. Futures can be implemented in Concurrent
Haskell using the primitive unsafeInterleaveIO, which is available in most implementations of Haskell. Our main result is conservativity
of CHF, that is, all equivalences of pure functional expressions are
also valid in CHF. This implies that compiler optimizations and transformations
from pure Haskell remain valid in Concurrent Haskell even if
it is extended by futures. We also show that this is no longer valid if Concurrent
Haskell is extended by the arbitrary use of unsafeInterleaveIO.
An abstract machine for concurrent Haskell with futures
- We show how Sestoft’s abstract machine for lazy evaluation
of purely functional programs can be extended to evaluate expressions of
the calculus CHF – a process calculus that models Concurrent Haskell
extended by imperative and implicit futures. The abstract machine is
modularly constructed by first adding monadic IO-actions to the machine
and then in a second step we add concurrency. Our main result is that
the abstract machine coincides with the original operational semantics
of CHF, w.r.t. may- and should-convergence.