TRANSLATE-PATHNAME accepts three additional keyword arguments: (TRANSLATE-PATHNAME source from-wildname to-wildname &KEY :ALL :MERGE :ABSOLUTE)

If :ALL is specified and non-NIL, a list of all resulting pathnames, corresponding to all matches of (PATHNAME-MATCH-P source from-wildname), is returned.

If :MERGE is specified and NIL, unspecified pieces of to-pathname are not replaced by corresponding pieces of source.

If :ABSOLUTE is specified and non-NIL, the returned pathnames are converted to absolute by merging in the current process' directory, therefore rendering pathnames suitable for the OS and external programs. So, to pass a pathname to an external program, you do (NAMESTRING (TRANSLATE-PATHNAME pathname #P"" #P"" :ABSOLUTE T)) or (NAMESTRING (EXT:ABSOLUTE-PATHNAME pathname)).

TRANSLATE-LOGICAL-PATHNAME accepts an additional keyword argument :ABSOLUTE, similar to Section 19.5.1, “Function TRANSLATE-PATHNAME”.

(PARSE-NAMESTRING string &OPTIONAL host defaults &KEY start end junk-allowed) returns a logical pathname only if host is a logical host or host is NIL and defaults is a LOGICAL-PATHNAME. To construct a logical pathname from a string, the function LOGICAL-PATHNAME can be used.

The [ANSI CL standard] behavior of recognizing logical pathnames when the string begins with some alphanumeric characters followed by a colon (#\:) can be very confusing (cf. "c:/autoexec.bat", "home:.clisprc" and "prep:/pub/gnu") and therefore is disabled by default. To enable the [ANSI CL standard] behavior, you should set CUSTOM:*PARSE-NAMESTRING-ANSI* to non-NIL. Note that this also disables treating SYMBOLs as pathname designators.

(MERGE-PATHNAMES pathname [default-pathname]) returns a logical pathname only if default-pathname is a LOGICAL-PATHNAME. To construct a logical pathname from a STRING, the function LOGICAL-PATHNAME can be used.

When both pathname and default-pathname are relative pathnames, the behavior depends on CUSTOM:*MERGE-PATHNAMES-ANSI*: when it is NIL, then CLISP retains its traditional behavior: (MERGE-PATHNAMES #P"x/" #P"y/") evaluates to #P"x/"

Rationale. MERGE-PATHNAMES is used to specify default components for pathnames, so there is some analogy between (MERGE-PATHNAMES a b) and (OR a b). Obviously, putting in the same default a second time should do the same as putting it in once: (OR a b b) is the same as (OR a b), so (MERGE-PATHNAMES (MERGE-PATHNAMES a b) b) should be the same as (MERGE-PATHNAMES a b).

(This question actually does matter because in Common Lisp there is no distinction between “pathnames with defaults merged-in” and “pathnames with defaults not yet applied”.)

Now, (MERGE-PATHNAMES (MERGE-PATHNAMES #P"x/" #P"y/") #P"y/") and (MERGE-PATHNAMES #P"x/" #P"y/") are EQUAL in CLISP (when CUSTOM:*MERGE-PATHNAMES-ANSI* is NIL), but not in implementations that strictly follow the [ANSI CL standard]. In fact, the above twice-default = once-default rule holds for all pathnames in CLISP.

Conversely, when CUSTOM:*MERGE-PATHNAMES-ANSI* is non-NIL, the normal [ANSI CL standard] behavior is exhibited: (MERGE-PATHNAMES #P"x/" #P"y/") evaluates to #P"y/x/".

Rationale. “merge” is merge and not or.

When the host argument to LOAD-LOGICAL-PATHNAME-TRANSLATIONS is not a defined logical host yet, we proceed as follows:

(EXT:ABSOLUTE-PATHNAME pathname) converts the pathname to a physical pathname, then - if its directory component is not absolute - converts it to an absolute pathname, by merging in the current process' directory. This is like TRUENAME, except that it does not verify that a file named by the pathname exists, not even that its directory exists. It does no filesystem accesses, except to determine the current directory. This function is useful when you want to save a pathname over time, or pass a pathname to an external program.