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#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 */
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