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