lix/src/fstate.hh
Eelco Dolstra 1d1c3691d2 * The policy-free derivate sharing now *almost* works. :-) For any
hash for which no local expansion is available, Nix can execute a
  `substitute' which should produce a path with such a hash.

  This is policy-free since Nix does not in any way specify how the
  substitute should work, i.e., it's an arbitrary (unnormalised)
  fstate expression.  For example, `nix-pull' registers substitutes
  that fetch Nix archives from the network (through `wget') and unpack
  them, but any other method is possible as well.  This is an
  improvement over the old Nix sharing scheme, which had a policy
  (fetching through `wget') built in.

  The sharing scheme doesn't work completely yet because successors
  from fstate rewriting have to be registered on the receiving side.
  Probably the whole successor stuff can be folded up into the
  substitute mechanism; this would be a nice simplification.
2003-07-10 15:11:48 +00:00

98 lines
3 KiB
C++

#ifndef __EVAL_H
#define __EVAL_H
#include <set>
extern "C" {
#include <aterm2.h>
}
#include "hash.hh"
using namespace std;
/* \section{Abstract syntax of Nix file system state expressions}
A Nix file system state expression, or FState, describes a
(partial) state of the file system.
Path : Path * Content * [FState] -> FState
Path(path, content, refs) specifies a file object (its full path
and contents), along with all file objects referenced by it (that
is, that it has pointers to). We assume that all files are
self-referential. This prevents us from having to deal with
cycles.
Derive : String * Path * [FState] * Path * [(String, String)] -> FState
Derive(platform, builder, ins, outs, env) specifies the creation of
new file objects (in paths declared by `outs') by the execution of
a program `builder' on a platform `platform'. This execution takes
place in a file system state given by `ins'. `env' specifies a
mapping of strings to strings.
[ !!! NOT IMPLEMENTED
Regular : String -> Content
Directory : [(String, Content)] -> Content
(this complicates unambiguous normalisation)
]
CHash : Hash -> Content
File content, given either in situ, or through an external reference
to the file system or url-space decorated with a hash to preserve
purity.
A FState expression is in {\em $f$-normal form} if all Derive nodes
have been reduced to File nodes.
DISCUSSION: the idea is that a Regular/Directory is interchangeable
with its CHash. This would appear to break referential
transparency, e.g., Derive(..., ..., [...CHash(h)...], ...) can
only be reduced in a context were the Regular/Directory equivalent
of Hash(h) is known. However, CHash should be viewed strictly as a
shorthand; that is, when we export an expression containing a
CHash, we should also export the file object referenced by that
CHash.
*/
typedef ATerm FState;
typedef ATerm Content;
typedef set<string> StringSet;
/* Realise an fstate expression in the file system. This requires
execution of all Derive() nodes. */
FState realiseFState(FState fs, StringSet & paths);
/* Return the path of an fstate expression. An empty string is
returned if the term is not a valid fstate expression. (!!!) */
string fstatePath(FState fs);
/* Return the paths referenced by fstate expression. */
void fstateRefs(FState fs, StringSet & paths);
/* Return a canonical textual representation of an expression. */
string printTerm(ATerm t);
/* Throw an exception with an error message containing the given
aterm. */
Error badTerm(const format & f, ATerm t);
/* Hash an aterm. */
Hash hashTerm(ATerm t);
FState hash2fstate(Hash hash);
/* Read an aterm from disk, given its hash. */
ATerm termFromHash(const Hash & hash, string * p = 0);
/* Write an aterm to the Nix store directory, and return its hash. */
Hash writeTerm(ATerm t, const string & suffix, string * p = 0);
#endif /* !__EVAL_H */