SWI-Prolog offers a query substitution mechanism similar to that of Unix csh (csh(1)), called `history'. It allows the user to compose new queries from those typed before and remembered by the system. It also allows to correct queries and syntax errors. SWI-Prolog does not offer the Unix csh capabilities to include arguments. This is omitted as it is unclear how the first, second, etc. argument should be defined. (3)
The available history commands are shown in table 1. Figure 1 gives some examples.
!!. | Repeat last query |
!nr. | Repeat query numbered <nr> |
!str. | Repeat last query starting with <str> |
!?str. | Repeat last query holding <str> |
^old^new. | Substitute <old> into <new> of last query |
!nr^old^new. | Substitute in query numbered <nr> |
!str^old^new. | Substitute in query starting with <str> |
!?str^old^new. | Substitute in query holding <str> |
h. | Show history list |
!h. | Show this list |
When in top level SWI-Prolog reads the user's queries using
read_history/6
rather than read/1.
This predicate first reads the current input stream up to a full stop.
While doing so it maps all contiguous blank space onto a single space
and deletes
/* ... */ and % ...<cr>
comments. Parts between double quotes (") or single quotes
(') are left unaltered. Note that a Prolog full stop
consists of a `non-symbol' character, followed by a period (.#$&*+-./:<=>?@^`~.
A single quote immediately preceded by a digit (0-9) is considered part
of the <digit>'<digit>... (e.g. 2'101;
binary number 101) sequence.
After this initial parsing the result is first checked for the
special
^<old>^<new>.
construction. If this fails the string is checked for all occurrences of
the !!?
From the above it follows that it is hard or impossible to correct quotation with single or double quotes, comment delimiters and spacing.