74 lines
2.2 KiB
C++
74 lines
2.2 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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/* \section{Abstract syntax of Nix file system state expressions}
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A Nix file system state expression, or FState, describes a
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(partial) state of the file system.
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File : Path * Content * [FState] -> FState
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File(path, content, refs) specifies a file object (its full path
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and contents), along with all file objects referenced by it (that
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is, that it has pointers to). We assume that all files are
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self-referential. This prevents us from having to deal with
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cycles.
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Derive : String * Path * [FState] * Path * [(String, String)] -> FState
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Derive(platform, builder, ins, outs, env) specifies the creation of
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new file objects (in paths declared by `outs') by the execution of
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a program `builder' on a platform `platform'. This execution takes
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place in a file system state given by `ins'. `env' specifies a
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mapping of strings to strings.
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[ !!! NOT IMPLEMENTED
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Regular : String -> Content
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Directory : [(String, Content)] -> Content
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(this complicates unambiguous normalisation)
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]
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CHash : Hash -> Content
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File content, given either in situ, or through an external reference
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to the file system or url-space decorated with a hash to preserve
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purity.
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A FState expression is in {\em $f$-normal form} if all Derive nodes
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have been reduced to File nodes.
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DISCUSSION: the idea is that a Regular/Directory is interchangeable
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with its CHash. This would appear to break referential
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transparency, e.g., Derive(..., ..., [...CHash(h)...], ...) can
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only be reduced in a context were the Regular/Directory equivalent
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of Hash(h) is known. However, CHash should be viewed strictly as a
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shorthand; that is, when we export an expression containing a
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CHash, we should also export the file object referenced by that
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CHash.
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*/
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typedef ATerm FState;
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typedef ATerm Content;
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/* Realise a $f$-normalised expression in the file system. */
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FState realiseFState(FState fs);
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/* Return a canonical textual representation of an expression. */
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string printTerm(ATerm t);
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/* Hash an aterm. */
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Hash hashTerm(ATerm t);
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#endif /* !__EVAL_H */
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