The Standard ML Basis Library

The `OS.Path` structure

Synopsis

signature OS_PATH structure Path : OS_PATH

The OS.Path structure provides support for manipulating the syntax of file system paths independent of the underlying file system. It is purposely designed not to rely on any file system operations: none of the functions accesses the actual file system. There are two reasons for this design: many systems support multiple file systems that may have different semantics and applications may need to manipulate paths that do not exist in the underlying file system.

Before discussing the model of paths and the semantics of the individual operations, we need to define some terms:

An arc denotes a directory or file relative to the directory in which it is recorded. In a path string, arcs are separated by the arc separator character. This character is #"/" in Unix in Microsoft Windows, #"\\" is used. This character is #"/" in Unix in Microsoft Windows, both #"\\" and #"/" are allowed. For example, in Unix, the path "abc/def" contains two arcs: "abc" and "def". There are two special arcs: parentArc and currentArc. Under both Unix and Microsoft Windows, the parentArc is ".." and currentArc is ".". An empty arc corresponds to an empty string.
Although represented concretely as a string, an arc should be viewed as an abstraction in the context of the OS.Path structure, with a limited set of valid representations. In particular, a non-empty string a corresponds to valid representation of an arc only if fromString a returns {isAbs=false, vol="", arcs=[a]}.
A path corresponds to a list of arcs, with an optional root, that denotes the path of directories leading to a file or directory in the file system hierarchy.
An absolute path has a root. Unix examples include "/" and "/a/b"; Microsoft Windows examples include "\", "\a\b", and "A:\a\b".
A relative path is one without a root. Unix examples include ".." and "a/b"; Microsoft Windows examples include "..", "a\b", and "A:a\b".
A canonical path contains no occurrences of the empty arc, no occurrences of the current arc unless the current arc is the only arc in the path, and contains parent arcs only at the beginning and only if the path is relative. Some examples of canonical paths, using Unix syntax, are as follows: "." "/.", "/", "a", "a/b/c", "..", "../a", "../../a/b/c", and "/a/b/c".
System note [WINDOWS]:

In a Microsoft Windows implementation, canonical paths are entirely lowercase.
A path has an associated volume. Under Unix, there is only one volume, whose name is "". Under Microsoft Windows, example volume names are "", "A:", and "C:".

In addition to operations for canonicalizing paths and computing relative paths, the Path structure supports path manipulations relative to three different views of a path:

A navigation oriented view, where a path is broken down into its root and a non-empty list of arcs. A path is either absolute or relative. The root of a path specifies the volume to which the path is taken to be relative. For Unix, there is only the "" volume.
A directory/file view, where a path is broken down into a directory specifier and a filename.
A base/extension view, where a path is broken down into a base filename and an extension. We make the assumption that the extension separator character is #".". This works for Microsoft Windows, OS/2, and Unix; the Macintosh does not really have a notion of extension.

Our main design principle is that the functions should behave in a natural fashion when applied to canonical paths. All functions, except concat, preserve canonical paths, i.e., if all arguments are canonical, then so is the result.

Note that although the model of path manipulation provided by the Path structure is operating system independent, the analysis of strings is not. In particular, any given implementation of the Path structure has an implicit notion of what the arc separator character is. Thus, on a Microsoft Windows system, Path will treat the string "\\d\\e" as representing an absolute path with two arcs, whereas on a Unix system, it will correspond to a relative path with one arc.

Interface

exception Path exception InvalidArc val parentArc : string val currentArc : string val fromString : string -> { isAbs : bool, vol : string, arcs : string list } val toString : { isAbs : bool, vol : string, arcs : string list } -> string val validVolume : {isAbs : bool, vol : string} -> bool val getVolume : string -> string val getParent : string -> string val splitDirFile : string -> {dir : string, file : string} val joinDirFile : {dir : string, file : string} -> string val dir : string -> string val file : string -> string val splitBaseExt : string -> {base : string, ext : string option } val joinBaseExt : {base : string, ext : string option} -> string val base : string -> string val ext : string -> string option val mkCanonical : string -> string val isCanonical : string -> bool val mkAbsolute : {path : string, relativeTo : string} -> string val mkRelative : {path : string, relativeTo : string} -> string val isAbsolute : string -> bool val isRelative : string -> bool val isRoot : string -> bool val concat : string * string -> string val fromUnixPath : string -> string val toUnixPath : string -> string

Description

val parentArc : string

The string denoting the parent directory (e.g., ".." on Microsoft Windows and Unix).

val currentArc : string

The string denoting the current directory (e.g., "." on Microsoft Windows and Unix).

fromString path

returns the decomposition {isAbs, vol, arcs} of the path specified by path. vol is the volume name and arcs is the list of (possibly empty) arcs of the path. isAbs is true if the path is absolute. Under Unix, the volume name is always the empty string; under Microsoft Windows, in addition it can have the form "A:", "C:", etc.

Here are some examples for Unix paths:

`path`	`fromString path`
`""`	`{isAbs=false, vol="", arcs=[]}`
`"/"`	`{isAbs=true, vol="", arcs=[""]}`
`"//"`	`{isAbs=true, vol="", arcs=["", ""]}`
`"a"`	`{isAbs=false, vol="", arcs=["a"]}`
`"/a"`	`{isAbs=true, vol="", arcs=["a"]}`
`"//a"`	`{isAbs=true, vol="", arcs=["","a"]}`
`"a/"`	`{isAbs=false, vol="", arcs=["a", ""]}`
`"a//"`	`{isAbs=false, vol="", arcs=["a", "", ""]}`
`"a/b"`	`{isAbs=false, vol="", arcs=["a", "b"]}`

toString {isAbs, vol, arcs}

makes a string out of a path represented as a list of arcs. isAbs specifies whether or not the path is absolute, and vol provides a corresponding volume. It returns "" when applied to {isAbs=false, vol="", arcs=[]}. The exception Path is raised if validVolume{isAbs, vol} is false, or if isAbs is false and arcs has an initial empty arc. The exception InvalidArc is raised if any component in arcs is not a valid representation of an arc. The exception Size is raised if the resulting string would have size greater than String.maxSize.

toString o fromString is the identity. fromString o toString is also the identity, provided no exception is raised and none of the strings in arcs contains an embedded arc separator character. In addition, isRelative(toString {isAbs=false, vol, arcs}) evaluates to true when defined.

validVolume {isAbs, vol}

returns true if vol is a valid volume name for an absolute or relative path, respectively as isAbs is true or false. Under Unix, the only valid volume name is "". Under Microsoft Windows, the valid volume names have the form "a:", "A:", "b:", "B:", etc. and, if isAbs = false, also "". Under MacOS, isAbs can be true if and only if vol is "".

getVolume path

returns the volume portion of the path path.

getParent path

returns a string denoting the parent directory of path. It holds that getParent path = path if and only if path is a root. If the last arc is empty or the parent arc, then getParent appends a parent arc. If the last arc is the current arc, then it is replaced with the parent arc. Note that if path is canonical, then the result of getParent will also be canonical.

Here are some examples for Unix paths:

`path`	`getParent path`
`"/"`	`"/"`
`"a"`	`"."`
`"a/"`	`"a/.."`
`"a///"`	`"a///.."`
`"a/b"`	`"a"`
`"a/b/"`	`"a/b/.."`
`".."`	`"../.."`
`"."`	`".."`
`""`	`".."`

splitDirFile path

splits the string path path into its directory and file parts, where the file part is defined to be the last arc. The file will be "", if the last arc is "".

Here are some examples for Unix paths:

`path`	`splitDirFile path`
`""`	`{dir = "", file = ""}`
`"."`	`{dir = "", file = "."}`
`"b"`	`{dir = "", file = "b"}`
`"b/"`	`{dir = "b", file = ""}`
`"a/b"`	`{dir = "a", file = "b"}`
`"/a"`	`{dir = "/", file = "a"}`

joinDirFile {dir, file}

creates a whole path out of a directory and a file by extending the path dir with the arc file. If the string file does not correspond to an arc, raises InvalidArc. The exception Size is raised if the resulting string would have size greater than String.maxSize.

dir path

file path

return the directory and file parts of a path, respectively. They are equivalent to #dir o splitDirFile and #file o splitDirFile, respectively, although they are probably more efficient.

splitBaseExt path

splits the path path into its base and extension parts. The extension is a non-empty sequence of characters following the right-most, non-initial, occurrence of "." in the last arc; NONE is returned if the extension is not defined. The base part is everything to the left of the extension except the final ".". Note that if there is no extension, a terminating "." is included with the base part.

Here are some examples for Unix paths:

`path`	`splitBaseExt path`
`""`	`{base = "", ext = NONE}`
`".login"`	`{base = ".login", ext = NONE}`
`"/.login"`	`{base = "/.login", ext = NONE}`
`"a"`	`{base = "a", ext = NONE}`
`"a."`	`{base = "a.", ext = NONE}`
`"a.b"`	`{base = "a", ext = SOME "b"}`
`"a.b.c"`	`{base = "a.b", ext = SOME "c"}`
`".news/comp"`	`{base = ".news/comp", ext = NONE}`

joinBaseExt {base, ext}

returns an arc composed of the base name and the extension (if different from NONE). It is a left inverse of splitBaseExt, i.e., joinBaseExt o splitBaseExt is the identity. The opposite does not hold, since the extension may be empty, or may contain extension separators. Note that although splitBaseExt will never return the extension SOME(""), joinBaseExt treats this as equivalent to NONE. The exception Size is raised if the resulting string would have size greater than String.maxSize.

base path

ext path

These return the base and extension parts of a path, respectively. They are equivalent to #base o splitBaseExt and #ext o splitBaseExt, respectively, although they are probably more efficient.

mkCanonical path

returns the canonical path equivalent to path. Redundant occurrences of the parent arc, the current arc, and the empty arc are removed. The canonical path will never be the empty string; the empty path is converted to the current directory path ("." under Unix and Microsoft Windows).

Note that the syntactic canonicalization provided by mkCanonical may not preserve file system meaning in the presence of symbolic links (see concat).

isCanonical path

returns true if path is a canonical path. It is equivalent to (path = mkCanonical path).

mkAbsolute {path, relativeTo}

returns an absolute path that is equivalent to the path path relative to the absolute path relativeTo. If path is already absolute, it is returned unchanged. Otherwise, the function returns the canonical concatenation of relativeTo with path, i.e., mkCanonical (concat (abs, p)). Thus, if path and relativeTo are canonical, the result will be canonical. If relativeTo is not absolute, or if the two paths refer to different volumes, then the Path exception is raised. The exception Size is raised if the resulting string would have size greater than String.maxSize.

mkRelative {path, relativeTo}

returns a relative path p that, when taken relative to the canonical form of the absolute path relativeTo, is equivalent to the path path. If path is relative, it is returned unchanged. If path is absolute, the procedure for computing the relative path is to first compute the canonical form abs of relativeTo. If path and abs are equal, then the current arc is the result. Otherwise, the common prefix is stripped from path and abs giving p' and abs'. The resulting path is then formed by appending p' to a path consisting of one parent arc for each arc in abs'. Note that if both paths are canonical, then the result will be canonical.

If relativeTo is not absolute, or if path and relativeTo are both absolute but have different roots, the Path exception is raised. The exception Size is raised if the resulting string would have size greater than String.maxSize.

Here are some examples for Unix paths:

`path`	`relativeTo`	`mkRelative{path, relativeTo}`
`"a/b"`	`"/c/d"`	`"a/b"`
`"/"`	`"/a/b/c"`	`"../../.."`
`"/a/b/"`	`"/a/c"`	`"../b/"`
`"/a/b"`	`"/a/c"`	`"../b"`
`"/a/b/"`	`"/a/c/"`	`"../b/"`
`"/a/b"`	`"/a/c/"`	`"../b"`
`"/"`	`"/"`	`"."`
`"/"`	`"/."`	`"."`
`"/"`	`"/.."`	`"."`
`"/a/b/../c"`	`"/a/d"`	`"../b/../c"`
`"/a/b"`	`"/c/d"`	`"../../a/b"`
`"/c/a/b"`	`"/c/d"`	`"../a/b"`
`"/c/d/a/b"`	`"/c/d"`	`"a/b"`

isAbsolute path

isRelative path

These return true if path is, respectively, absolute or relative.

isRoot path

returns true if path is a canonical specification of a root directory.

concat (path, t)

returns the path consisting of path followed by t. It raises the exception Path if t is not a relative path or if path and t refer to different volumes. The exception Size is raised if the resulting string would have size greater than String.maxSize.

A implementation of concat might be:

fun concat (p1, p2) = (case (fromString p1, fromString p2)
       of (_, {isAbs=true, ...}) => raise Path
        | ({isAbs, vol=v1, arcs=al1},
           {vol=v2, arcs=al2, ...}
          ) => if ((v2 = "") orelse (v1 = v2))
              then toString{
                  isAbs=isAbs, vol=v1,
                  arcs=concatArcs(al1, al2)
                }
              else raise Path
      (* end case *))

where concatArcs is like List.@, except that a trailing empty arc in the first argument is dropped. Note that concat should not be confused with the concatenation of two strings.

concat does not preserve canonical paths. For example, concat("a/b", "../c") returns "a/b/../c". The parent arc is not removed because "a/b/../c" and "a/c" may not be equivalent in the presence of symbolic links.

fromUnixPath s

converts the Unix-style path s to the path syntax of the host operating system. Slash characters are translated to the directory separators of the local system, as are parent arcs and current arcs. This function raises the InvalidArc exception if any arc in the Unix path is invalid in the host system's path syntax (e.g., an arc that has a backslash character in it when the host system is Microsoft Windows).

Note that the syntax of Unix pathnames necessarily limits this function. It is not possible to specify paths that have a non-empty volume name or paths that have a slash in one of their arcs using this function.

toUnixPath s

converts the path s, which is in the host operating system's syntax, to a Unix-style path. If the path s has a non-empty volume name, then the Path exception is raised. Also, if any arc in the pathname contains the slash character, then the InvalidArc exception is raised.

Discussion

Syntactically, two paths can be checked for equality by applying string equality to canonical versions of the paths. Since volumes and individual arcs are just special classes of paths, an identical test for equality can be applied to these classes.

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Generated April 12, 2004
Last Modified June 18, 2002
Comments to John Reppy.

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The Standard ML Basis Library

The OS.Path structure

Synopsis

Interface

Description

See Also

Discussion

The `OS.Path` structure