source: trunk/abcl/doc/design/pathnames/merging-defaults.markdown @ 14174

Last change on this file since 14174 was 14174, checked in by Mark Evenson, 8 years ago

Note why Windows doesn't need the proposed merge PATHNAME changes.

File size: 3.5 KB
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1# ISSUE MERGE-PATHNAMES with specialization of JAR-PATHNAME
2
3We wish to resolve the following issues.
4
5## UC0 Loading jna.jar for CFFI via Quicklisp
6
7Currently we cannount load systems via ASDF recursively in jars, most
8importantly for those packaged in 'abcl-contrib.jar', which prevents
9CFFI from loading the necessary JNA code.
10
11## UC1.1
12If *DEFAULT-PATHNAME-DEFAULTS* is a JAR-PATHNAME, commands that would
13normally be expected to work such as
14
15    CL-USER> (probe-file #p"/")
16   
17will fail. 
18
19### UC1.2
20
21If *DEFAULT-PATHNAME-DEFAULTS* is a JAR-PATHNAME, COMPILE-FILE
22operations specifying an OUTPUT-FILE with a NIL DEVICE will fail, as
23COMPILE-FILE-PATHNAME is required to merge its arguments with the
24defaults.
25
26## CLHS Citations
27
28Some especially relevant portions of the CLHS for consideration.
29
30###  19.2.3 Merging Pathnames
31http://www.lispworks.com/documentation/HyperSpec/Body/19_bc.htm
32
33"Except as explicitly specified otherwise, for functions that
34manipulate or inquire about files in the file system, the pathname
35argument to such a function is merged with *default-pathname-defaults*
36before accessing the file system (as if by merge-pathnames)."
37
38Note that this implies that the arguments to PARSE-NAMESTRING--which
39is what the SHARSIGN-P reader macro correpsponds to--should not be
40merged.
41
42
43## 19.2.2.2.3 :UNSPECIFIC as a Component Value
44http://www.lispworks.com/documentation/HyperSpec/Body/19_bbbc.htm
45
46"If :unspecific is the value of a pathname component, the component is
47considered to be ``absent'' or to ``have no meaning'' in the filename
48being represented by the pathname."
49
50Having an :UNSPECIFIC DEVICE when a PATHNAME refers to a file would
51address problems when merging when the defaults contains a JAR-PATHNAME.
52
53### MERGE-PATHNAMES
54http://www.lispworks.com/documentation/HyperSpec/Body/f_merge_.htm
55
56"If pathname explicitly specifies a host and not a device, and if the
57host component of default-pathname matches the host component of
58pathname, then the device is taken from the default-pathname;
59otherwise the device will be the default file device for that host. If
60pathname does not specify a host, device, directory, name, or type,
61each such component is copied from default-pathname."
62
63This suggests that the contents HOST should be considered as an
64additional axis of type for PATHNAME, so that when PATHNAMES of
65differing types get merged, a DEVICE which has no meaning for a given
66type does not get inserted.
67
68## Other implementations
69
70A survey of the the "default" host for #p"/" on startup.
71
72### SBCL
73
74A host nonce which appears in the reader as
75#<SB-IMPL::UNIX-HOST {1000290443}>.  (Is there a different one under
76Windows?)
77
78### CLISP
79
80HOST is NIL.
81
82### CCL
83
84HOST is :UNSPECIFIC.
85
86### ECL
87
88HOST is NIL.
89
90## Implementation
91
92Since Windows systems do have a default DEVICE for a normal file
93PATHNAME, namely the current "drive letter" of the process, the
94implementation changes will be mostly wrapped in runtime conditionals
95for non-Windows systems.
96
97### TRUENAME sets DEVICE to :UNSPECIFIC
98
99TRUENAME sets DEVICE to :UNSPECIFIC running on non-Windows when
100resolving a path to a plain file.
101
102### Use an implicit type for merging
103
104In the case for which a merge occurs when DEFAULT-PATHNAME
105is a JAR-PATHNAME and the following conditions hold:
106
1071.  HOST and DEVICE of the PATHNAME are NIL
108
1092.  The DIRECTORY of the PATHNAME represents an absolute path.
110
1113.  We are not on Windows.
112
113we set the DEVICE to be :UNSPECIFIC.
114
115### COLOPHON
116
117Mark <evenson@panix.com>
118Created:  01-SEP-2012
119Revised:  09-OCT-2012
120
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