[Haskell-cafe] Modelling a mutable variable store
derek.a.elkins at gmail.com
Sat Dec 1 22:22:53 EST 2007
On Sun, 2007-12-02 at 03:11 +0000, Kannan Goundan wrote:
> I'm implementing an interpreter for the lambda calculus augmented with
> mutable variables. I'm having problems doing the mutable state stuff in
> Haskell. Here's what I have so far:
> type Expr = ... terms in the language ...
> type Value = ... values in the language ...
> type Ident = String
> type VarId = String
> type Env = Map Ident Target
> data Target = TValue Value | TVar VarId
> -- idents might refer to values or vars in the store.
> type Store = Map VarId (Maybe Value)
> -- Using "Maybe" because vars start out uninitialized.
> eval :: (Env, Store, Expr) -> (Store, Value)
> -- The Store is threaded through the evaluator.
> One problem is that the link between an identifier and it's entry in the
> store is via VarIds, which are just strings. For example, if I see the
> identifier "x", I first look it up in the Env, and if it is a "TVar", I
> then look it up in the store. I'd like to get something stronger (in
> Java, I would use a pointer).
> Another problem is that entries in my Store never get garbage collected.
> Again, if I were using pointers in Java, this wouldn't be an issue.
> There's also the issue of finding a more elegant way of threading the
> Store through my evaluator, but I'm not concerned too much about that at
> this point. I can probably define a state-carrying monad like Parsec.
> My real concerns are the first two issues.
Use ST. First-class state isn't too great unless you specifically want
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