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#ifndef __EVAL_H
#define __EVAL_H
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;
/* Realise a $f$-normalised expression in the file system. */
FState realiseFState(FState fs);
/* 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);
/* Write an aterm to the Nix store directory, and return its hash. */
Hash writeTerm(ATerm t);
#endif /* !__EVAL_H */
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