#ifndef __EVAL_H
#define __EVAL_H
extern "C" {
#include <aterm2.h>
}
#include "hash.hh"
using namespace std;
/* Abstract syntax of Nix values:
e := Deref(e) -- external expression
| Hash(h) -- value reference
| Str(s) -- string constant
| Bool(b) -- boolean constant
| Var(x) -- variable
| App(e, e) -- application
| Lam(x, e) -- lambda abstraction
| Exec(platform, e, [Arg(e, e)])
-- primitive; execute e with args e* on platform
;
TODO: Deref(e) allows computed external expressions, which might be
too expressive; perhaps this should be Deref(h).
Semantics
Each rule given as eval(e) => e', i.e., expression e has a normal
form e'.
eval(Deref(Hash(h))) => eval(loadExpr(h))
eval(Hash(h)) => Hash(h) # idem for Str, Bool
eval(App(e1, e2)) => eval(App(e1', e2))
where e1' = eval(e1)
eval(App(Lam(var, body), arg)) =>
eval(subst(var, arg, body))
eval(Exec(platform, prog, args)) => Hash(h)
where
fn = ... name of the output (random or by hashing expr) ...
h =
if exec( fn
, eval(platform) => Str(...)
, getFile(eval(prog))
, map(makeArg . eval, args)
) then
hashPath(fn)
else
undef
... register ...
makeArg(Arg(Str(nm), (Hash(h), h))) => (nm, getFile(h))
makeArg(Arg(Str(nm), (Str(s), _))) => (nm, s)
makeArg(Arg(Str(nm), (Bool(True), _))) => (nm, "1")
makeArg(Arg(Str(nm), (Bool(False), _))) => (nm, undef)
subst(x, e1, e2) is defined as a generic topdown term
traversal of e2, replacing each `Var(x)' with e1, and not
descending into `Lam(x, _)'.
Note: all stored expressions must be closed. !!! ugly
getFile :: Hash -> FileName
loadExpr :: Hash -> FileName
hashExpr :: Expr -> Hash
hashPath :: FileName -> Hash
exec :: FileName -> Platform -> FileName -> [(String, String)] -> Status
*/
typedef ATerm Expr;
/* Evaluate an expression. */
Expr evalValue(Expr e);
/* Return a canonical textual representation of an expression. */
string printExpr(Expr e);
/* Perform variable substitution. */
Expr substExpr(string x, Expr rep, Expr e);
/* Hash an expression. */
Hash hashExpr(Expr e);
#endif /* !__EVAL_H */
