Technical report Frank / Johann-Wolfgang-Goethe-Universität, Fachbereich Informatik und Mathematik, Institut für Informatik
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 We also show that this is no longer valid if Concurrent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.
A contextual semantics for concurrent Haskell with futures
- In this paper we analyze the semantics of a higher-order functional language with concurrent threads, monadic IO and synchronizing variables as in Concurrent Haskell. To assure declarativeness of concurrent programming we extend the language by implicit, monadic, and concurrent futures. As semantic model we introduce and analyze the process calculus CHF, which represents a typed core language of Concurrent Haskell extended by concurrent futures. Evaluation in CHF is defined by a small-step reduction relation. Using contextual equivalence based on may- and should-convergence as program equivalence, we show that various transformations preserve program equivalence. We establish a context lemma easing those correctness proofs. An important result is that call-by-need and call-by-name evaluation are equivalent in CHF, since they induce the same program equivalence. Finally we show that the monad laws hold in CHF under mild restrictions on Haskell’s seq-operator, which for instance justifies the use of the do-notation.