* Started implementing the new evaluation model.

* Lots of refactorings.
* Unit tests.

src/eval.hh

@@ -0,0 +1,86 @@
+#ifndef __EVAL_H
+#define __EVAL_H
+
+extern "C" {
+#include <aterm2.h>
+}
+
+#include "hash.hh"
+
+using namespace std;
+
+
+/* Abstract syntax of Nix values:
+
+   e := Hash(h) -- reference to expression value
+      | External(h) -- reference to non-expression value
+      | Str(s) -- string constant
+      | Bool(b) -- boolean constant
+      | App(e, e) -- application
+      | Lam(x, e) -- lambda abstraction
+      | Exec(platform, e, [(s, e)])
+          -- primitive; execute e with args e* on platform
+      ;
+
+   Semantics
+
+   Each rules given as eval(e) => (e', h'), i.e., expression e has a
+   normal form e' with hash code h'.  evalE = fst . eval.  evalH = snd
+   . eval.
+
+   eval(Hash(h)) => eval(loadExpr(h))
+
+   eval(External(h)) => (External(h), h)
+
+   eval(Str(s)@e) => (e, 0) # idem for Bool
+
+   eval(App(e1, e2)) => eval(App(e1', e2))
+     where e1' = evalE(e1)
+
+   eval(App(Lam(var, body), arg)@in) =>
+     eval(subst(var, arg, body))@out
+     [AND write out to storage, and dbNFs[hash(in)] = hash(out) ???]
+
+   eval(Exec(platform, prog, args)@e) =>
+     (External(h), h)
+     where
+       hIn = hashExpr(e)
+
+       fn = ... form name involving hIn ...
+
+       h =
+         if exec(evalE(platform) => Str(...)
+                , getFile(evalH(prog))
+                , map(makeArg . eval, args)
+                ) then
+           hashExternal(fn)
+         else
+           undef
+
+   makeArg((argn, (External(h), h))) => (argn, getFile(h))
+   makeArg((argn, (Str(s), _))) => (argn, s)
+   makeArg((argn, (Bool(True), _))) => (argn, "1")
+   makeArg((argn, (Bool(False), _))) => (argn, undef)
+
+   getFile :: Hash -> FileName
+   loadExpr :: Hash -> FileName
+   hashExpr :: Expr -> Hash 
+   hashExternal :: FileName -> Hash
+   exec :: Platform -> FileName -> [(String, String)] -> Status
+*/
+
+typedef ATerm Expr;
+
+
+struct EvalResult 
+{
+    Expr e;
+    Hash h;
+};
+
+
+/* Evaluate an expression. */
+EvalResult evalValue(Expr e);
+
+
+#endif /* !__EVAL_H */