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2.2 The GNU Prolog interactive interpreter

2.2.1 Starting/exiting the interactive interpreter
GNU Prolog offers a classical Prolog interactive interpreter also called top-level. It allows the user to execute queries, to consult Prolog programs, to list them, to execute them and to debug them. The top-level can be invoked using the following command:

% gprolog    (the % symbol is the operating system shell prompt)
The top-level invocation does not need any supplemental line-command option. When present these options can be retrieved using argument_value/2 (section 6.27.2) or argument_list/1 (section 6.27.3). However, the --verbose and --help options are recognized for compatibility purpose with other GNU programs.

To quit the top-level type the end-of-file key sequence (Ctl-D) or its term representation: end_of_file. It is also possible to use the built-in predicate halt/0 (section 6.18.1).

2.2.2 The interactive interpreter read-execute-write loop
The GNU Prolog top-level is built on a classical read-execute-write loop that also allows for re-executions (when the query is not deterministic) as follows:

Here is an example of execution of a query (``find the lists X and Y such that the concatenation of X and Y is [a,b]''):

| ?- append(X,Y,[a,b,c]).
 
X = []
Y = [a,b,c] ?    (here the user presses ; to compute another solution)
 
X = [a]
Y = [b,c] ?    (here the user presses a to compute all remaining solutions)
 
X = [a,b]
Y = [c]    (here the user is not asked and the next solution is computed)
 
X = [a,b,c]
Y = []    (here the user is not asked and the next solution is computed)
 
no    (no more solution)
In some cases the top-level can detect that the current solution is the last one (no more alternatives remaining). In such a case it does not display the ? symbol (and does not ask the user). Example:

| ?- (X=1 ; X=2).
 
X = 1 ?    (here the user presses ; to compute another solution)
 
X = 2    (here the user is not prompted since there are no more alternatives)
 
yes
The user can stop the execution even if there are more alternatives by typing RETURN.

| ?- (X=1 ; X=2).
 
X = 1 ?    (here the user presses RETURN to stop the execution)
 
yes
The top-level tries to display the values of the variables of the query in a readable manner. For instance, when a variable is bound to a query variable, the name of this variable appears. When a variable is a singleton an underscore symbol _ is displayed (_ is a generic name for a singleton variable, it is also called an anonymous variable). Other variables are bound to new brand variable names. When a query variable name X appears as the value of another query variable Y it is because X is itself not instantiated otherwise the value of X is displayed. In such a case, nothing is output for X itself (since it is a variable). Example:

| ?- X=f(A,B,_,A), A=k.
 
A = k    (the value of A is displayed also in f/3 for X)
X = f(k,B,_,k)    (since B is a variable which is also a part of X, B is not displayed)
| ?- functor(T,f,3), arg(1,T,X), arg(3,T,X).
 
T = f(X,_,X)    (the 1st and 3rd args are equal to X, the 2nd is an anonymous variable)
| ?- read_from_atom('k(X,Y,X).',T).
 
T = k(A,_,A)    (the 1st and 3rd args are unified, a new variable name A is introduced)
The top-level uses variable binding predicates (section 6.5). To display the value of a variable, the top-level calls write_term/3 with the following option list: [quoted(true),numbervars(false), namevars(true)] (section 6.14.6). A term of the form '$VARNAME'(Name) where Name is an atom is displayed as a variable name while a term of the form '$VAR'(N) where N is an integer is displayed as a normal compound term (such a term could be output as a variable name by write_term/3). Example:

| ?- X='$VARNAME'('Y'), Y='$VAR'(1).
 
X = Y    (the term '$VARNAME'('Y') is displayed as Y)
Y = '$VAR'(1)    (the term '$VAR'(1) is displayed as is)
| ?- X=Y, Y='$VAR'(1).
 
X = '$VAR'(1)
Y = '$VAR'(1)
In the first example, X is explicitly bound to '$VARNAME'('Y') by the query so the top-level displays Y as the value of X. Y is unified with '$VAR'(1) so the top-level displays it as a normal compound term. It should be clear that X is not bound to Y (whereas it is in the second query). This behavior should be kept in mind when doing variable binding operations.

Finally, the top-level computes the user-time (section 6.24.2) taken by a query and displays it when it is significant. Example:

| ?- retractall(p(_)), assertz(p(0)),
     repeat,
        retract(p(X)),
        Y is X+1,
        assertz(p(Y)),
        X=1000, !.
 
X = 1000
Y = 1001
 
(180 ms) yes    (the query took 180ms of user time)
2.2.3 Consulting a Prolog program
The top-level allows the user to consult Prolog source files. Consulted predicates can be listed, executed and debugged (while predicates compiled to native-code cannot). For more information about the difference between a native-code predicate and a consulted predicate refer to the introduction of this section (section 2.1) and to the part devoted to the compiler (section 2.4.1).

To consult a program use the built-in predicate consult/1 (section 6.23.1). The argument of this predicate is a Prolog file name or user to specify the terminal. This allows the user to directly input the predicates from the terminal. In that case the input shall be terminated by the end-of-file key sequence (Ctl-D) or its term representation: end_of_file. A shorthand for consult(FILE) is [FILE]. Example:

| ?- [user].
{compiling user for byte code...}
even(0).
even(s(s(X))):-
        even(X).
     (here the user presses Ctl-D to end the input)
{user compiled, 3 lines read - 350 bytes written, 1180 ms}
 
| ?- even(X).
 
X = 0 ?    (here the user presses ; to compute another solution)
 
X = s(s(0)) ?    (here the user presses ; to compute another solution)
 
X = s(s(s(s(0)))) ?    (here the user presses RETURN to stop the execution)
 
yes
| ?- listing.
 
even(0).
even(s(s(A))) :-
        even(A).
When consult/1 (section 6.23.1) is invoked on a Prolog file it first runs the GNU Prolog compiler (section 2.4) as a child process to generate a temporary WAM file for byte-code. If the compilation fails a message is displayed and nothing is loaded. If the compilation succeeds, the produced file is loaded into memory using load/1 (section 6.23.2). Namely, the byte-code of each predicate is loaded. When a predicate P is loaded if there is a previous definition for P it is removed (i.e. all clauses defining P are erased). We say that P is redefined. Note that only consulted predicates can be redefined. If P is a native-code predicate, trying to redefine it will produce an error at load-time: the predicate redefinition will be ignored and the following message displayed:

native code procedure P cannot be redefined
Finally, an existing predicate will not be removed if it is not re-loaded. This means that if a predicate P is loaded when consulting the file F, and if later the definition of P is removed from the file F, consulting F again will not remove the previously loaded definition of P from the memory.

Consulted predicates can be debugged using the Prolog debugger. Use the debugger predicate trace/0 or debug/0 (section 3.3.1) to activate the debugger.

2.2.4 Interrupting a query
Under the top-level it is possible to interrupt the execution of a query by typing the interruption key (Ctl-C). This can be used to abort a query, to stop an infinite loop, to activate the debugger,...When an interruption occurs the top-level displays the following message: Prolog interruption (h for help) ? The user can then type one of the following commands:

Command Name Description
a abort abort the current execution. Same as abort/0 (section 6.18.1)
e exit quit the current Prolog process. Same as halt/0 (section 6.18.1)
b break invoke a recursive top-level. Same as break/0 (section 6.18.1)
c continue resume the execution
t trace start the debugger using trace/0 (section 3.3.1)
d debug start the debugger using debug/0 (section 3.3.1)
h or ? help display a summary of available commands

2.2.5 The line editor
The line editor (linedit) allows the user to build/update the current input line using a variety of commands. This facility is available if the linedit part of GNU Prolog has been installed. linedit is implicitly called by any built-in predicate reading from a terminal (e.g. get_char/1, read/1,...). This is the case when the top-level reads a query.

Bindings: each command of linedit is activated using a key. For some commands another key is also available to invoke the command (on some terminals this other key may not work properly while the primary key always works). Here is the list of available commands:

Key Alternate key Description
Ctl-B ¬ go to the previous character
Ctl-F ® go to the next character
Esc-B Ctl-¬ go to the previous word
Esc-F Ctl-® go to the next word
Ctl-A Home go to the beginning of the line
Ctl-E End go to the end of the line
Ctl-H Backspace delete the previous character
Ctl-D Delete delete the current character
Ctl-U Ctl-Home delete from beginning of the line to the current character
Ctl-K Ctl-End delete from the current character to the end of the line
Esc-L   lower case the next word
Esc-U   upper case the next word
Esc-C   capitalize the next word
Ctl-T   exchange last two characters
Ctl-V Insert switch on/off the insert/replace mode
Ctl-I Tab complete word (twice displays all possible completions)
Ctl-space   mark beginning of the selection
Esc-W   copy (from the begin selection mark to the current character)
Ctl-W   cut (from the begin selection mark to the current character)
Ctl-Y   paste
Ctl-P ­ recall previous history line
Ctl-N ¯ recall next history line
Esc-P   recall previous history line beginning with the current prefix
Esc-N   recall next history line beginning with the current prefix
Esc-< Page Up recall first history line
Esc-> Page Down recall last history line
Ctl-C   generate an interrupt signal (section 2.2.4)
Ctl-D   generate an end-of-file character (at the begin of the line)
RETURN   validate a line
Esc-?   display a summary of available commands

History: when a line is entered (i.e. terminated by RETURN), linedit records it in an internal list called history. It is later possible to recall history lines using appropriate commands (e.g. Ctl-P recall the last entered line) and to modify them as needed. It is also possible to recall a history line beginning with a given prefix. For instance to recall the previous line beginning with write simply type write followed by Esc-P. Another Esc-P will recall an earlier line beginning with write,...

Completion: another important feature of linedit is its completion facility. Indeed, linedit maintains a list of known words and uses it to complete the prefix of a word. Initially this list contains all predefined atoms and the atoms corresponding to available predicates. This list is dynamically updated when a new atom appears in the system (whether read at the top-level, created with a built-in predicate, associated to a new consulted predicate,...). When the completion key (Tab) is pressed linedit acts as follows:

Example:

| ?- argu    (here the user presses Tab to complete the word)
| ?- argument_    (linedit completes argu with argument_ and emits a beep)
     (the user presses again Tab to see all possible completions)
argument_counter    (linedit shows 3 possible completions)
argument_list
argument_value
| ?- argument_    (linedit redisplays the input line)
 
| ?- argument_c    (to select argument_counter the user presses c and Tab)
| ?- argument_counter    (linedit completes with argument_counter)
Finally, linedit allows the user to check that (square/curly) brackets are well balanced. For this, when a close bracket symbol, i.e. ), ] or }, is typed, linedit determines the associated open bracket, i.e. (, [ or {, and temporarily repositions the cursor on it to show the match.

Copyright (C) 1999,2000 Daniel Diaz

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