[Haskell-cafe] small step evaluation as an unfold?

John Meacham john at repetae.net
Tue Jan 23 22:51:26 EST 2007

On Tue, Jan 23, 2007 at 10:25:27PM -0500, Steve Downey wrote:
> (overall context - working through TaPL on my own, reimplemnting
> typecheckers in haskell)
> the type checkers all follow the same pattern, in ocaml they throw an
> exception when the small step fails, which may mean taking another
> branch in the eval, but that that sub expression has hit bottom.
> it is self admittedly not good ocaml, and it seems to be even worse
> haskell, as i try to extend the simple evaluator i have to deal with
> managing reporting errors.
> having the single small step evaluator return a Maybe is fairly close.
> then the evaluator above it just bottoms out when eval1 expr returns
> Nothing, by passing expr back up as the result.
> but it occurs to me that it might be better to express it as an
> unfold, where the result is a list with the last element as the
> irresucible expression.

you would probably be interested in the helium type checker, which is
designed to give excellent error messages above all else. Basically,
what it does (up to my understanding) is perform a standard
type-inference traversal of your code, but rather than unify things as
it comes to them, it collects a set of constraints of what to unify with
what. then, once they are all collected, it can use a variety of
constraint solving techniques, whichever produces the best messages. it
even allows users to annotate their routines with specialized constraint
solving strategies and type error messages. it is really quite neat.



John Meacham - ⑆repetae.net⑆john⑈

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