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Tips and Tricks

Tips on using SPIDER.


Right-click inside a Prolog editor brings up a menu with commands; right-click in the its left margin will bring up a different set of commands, e.g. related to breakpoints. Additional commands are available from the menu bar.

It is possible to add key bindings for commands. This is an Eclipse feature, available from the Preferences. This way you can add a key binding for e.g. the “Consult Prolog Source” command, if desired.

The following summarizes some especially useful or powerful commands.


Go to definition

CTRL-mouse click (⌘-mouse click on macOS) on a predicate name will go to its definition. This also works for symbolic paths, e.g. the first argument to use_module/2 and variables. Alternatively, you can use the F3 keyboard command.

Open predicate

CTRL-O (⌘-O on macOS) opens a dialog where the name of a predicate can be entered and its definition opened.

Select enclosing clause

CTRL-SHIFT-A (⌘-SHIFT-A on macOS) will select the enclosing clause or directive. Repeated application selects successively larger units (associated comments, predicate, etc.). There are many other Prolog-specific commands for expanding the selection in the “Edit” menu.

Go to caller

CTRL-ALT-SHIFT-H when in a clause will go to the closest caller of the current predicate. Repeated invocations thus navigate backward through the call hierarchy.

Open Call Hierarchy

CTRL-ALT-H will open a tree of callers and other references to the predicate. This also works for symbolic paths and will tell you who loads a particular file, and which predicates in a file are used from outside the file.

Prolog Search

CTRL-^ searches for terms subsumed by a pattern. For instance, foo(_,_) would search for all occurences of a compound term with name foo and two arguments (in this example it would also find occurrences where the source code uses infix notation like a foo 42).

Refactoring and Code Formatting

Correct Indentation

CTRL-I will adjust the indent of the current line.

Format Element

CTRL-SHIFT-F (⌘-SHIFT-F on macOS) will format all lines of the enclosing clause or directive. If there is a selection, it will only format the selection.

In the settings you can change some details of how code is formatted.

Some formatting settings are also read from any -*- ... -*- line at the start of the the Prolog file. To see the supported variables, use the “New Prolog File” wizard; it will create a file with all supported Emacs variables filled in from the settings. The “New Prolog File” wizard can be accessed from the File menu, under New/Other/Prolog/Prolog File.

SPIDER reads Emacs-style Per-Directory Local Variables. This makes it easy to ensure consistent formatting between developers, regardless of whether they use Emacs or SPIDER, without modifying any source files.

A file, named .dir-locals.el in the same (or an ancestor-) directory as a Prolog file will be used for settings that are not present as a -*- ... -*- line in the Prolog file. E.g.

;; This should go in a .dir-locals.el file
 (prolog-mode .
               ;; These correspond to the SPIDER defaults
               (indent-tabs-mode . nil)
               (tab-width . 8)
               (prolog-indent-width . 8)
               (prolog-paren-indent . 4)
               (comment-column . 32)
Rename Predicate/Variable

CTRL-ALT-R when the cursor is on a goal or a variable brings up a dialog where a new name can be entered. When a predicate is renamed, all references in the project, not just those in the current file, will be updated by default.

Change Predicate Signature

CTRL-ALT-C when the cursor is on a predicate name brings up a dialog where arguments can be reordered, and arguments can be added or removed. When a predicate is changed, all references in the project, not just those in the current file, will be updated by default.

Introduce Predicate

CTRL-ALT-M (⌘-⌥-M on macOS) when one or more goals in a clause body are selected, will replace the selected goals with a call to a new predicate defined using the selected goals. Inline, when the cursor is at a call to a predicate, will do the reverse, i.e. replace the call with the body of the called predicate.

Introduce Variable

CTRL-ALT-V (⌘-⌥-V on macOS) will replace a selected term with a variable bound to the term. Inline, when the cursor is at a variable, will do the reverse, i.e. replace a variable with its value.

Inline Predicate/Variable

CTRL-ALT-I (⌘-⌥-I macOS) when the cursor is at a call to a predicate, or at a variable, will do the reverse of Introduce Predicate or Introduce Variable, depending on what is at the cursor, i.e. replace the call with the body of the predicate, or the variable with its value.

Renumber Variables

Renumbers the selected variable and other similarly named (Var, Var47, …) to be numbered consecutively. As an example,

foo(a, p(X42), X3) :-
        body(a, X, X).

would become:

foo(a, p(X1), X) :-
        body(a, X2, X2).
Linearize Arguments

Ensures that each argument position is a variable that does not occur elsewhere in the clause head. As an example,

foo(a, p(X1), X) :-
        body(a, X2, X2).

would become:

foo(A, A1, X) :-
        A = a,
        A1 = p(X1),
        body(a, X2, X2).
Introduce Module Directive

will convert a non-module file to a module by inserting a module/2 directive in the file. The module directive will have an export list containing suitable predicates.

Prolog Top Level

Compile Prolog Source

ALT-K will ask to save any modified files and then compile the edited file in the running SICStus.

Open Prolog Toplevel

CTRL-ALT-Q C (also available from the “SICStus” menu) will open the SICStus top level view.

Restart Prolog

To start or re-start SICStus in the Toplevel view, click on the the “Restart Prolog” (Restart icon) button.

Top level menu

The top level view has some useful commands in the "view menu", available under the view menu (View menu icon) button. Among other things, you can change the working directory of the running Prolog instance.

Top level completion

In the top level view Word Completion (ALT-/, CTRL-.) will complete the current word, like in the Prolog editor. Word completion is based on the text in the top level and the open Prolog editors. Predicate Completion (CTRL-SPACE) will complete predicate names, like in the Prolog editor. The history navigation History Previous (CTRL+SHIFT+UP-ARROW) and History Next (CTRL+SHIFT+DOWN-ARROW) will only show history entries that match the already typed text.

Debugging and Profiling

Create break point

Double-clicking in the left margin will create a spypoint or a line breakpoint. Adding a breakpoint to the first line of a predicate will create a spypoint.

Note that line breakpoints only work if source info is enabled, i.e. with set_prolog_flag(source_info, on).

Breakpoints are persisted and automatically installed in any Prolog started from within SPIDER. You can temporarily disable all breakpoints from the Breakpoints view.

Open Profile

“Open Profile”, available from the “SICStus” menu, will open the Profile view showing profiling info. Use set_prolog_flag(profiling, on) in Prolog to make Prolog collect profiling information.

Open Coverage

“Open Coverage”, available from the “SICStus” menu, will open the Coverage view showing source code coverage information. Source code coverage information is collected whenever profiling information is collected, i.e. when set_prolog_flag(profiling, on) is in effect. When SPIDER presents source code coverage in the Coverage view it will also show coverage information in the left margin of any affected source files.

Code Structure

SPIDER should be able to do a god job for any SICStus-compatible source code (and probably also for code written using other Prolog dialects). However, it will work best if some conventions are followed, as outlined below.

  • SPIDER works best when each file defines a module and where no module spans multiple files.

  • SPIDER works best with self-contained code. That is, code that defines one module per file and that brings in all its dependencies using use_module/[1,2] etc.

  • SPIDER cannot expand code with goal_expansion or term_expansion except for special cases like Definite Clause Grammars.

  • Avoid conjunctions of directives, e.g. do not use:
    :- op(foo,xfy,800), op(bar, fx, 10).
    instead use separate directives, like:
    :- op(foo,xfy,800).
    :- op(bar, fx, 10).
    This is required also for ISO compliance.
  • SPIDER knows about conditional compilation directives but has limited knowledge about whether a condition is true or not. It will warn if it encounters a condition it is unable to evaluate. In the context of a conditional directive the Prolog editor will handle at least the following goals:
    • current_prolog_flag(dialect, spider) is treated as true. This can be used to wrap code that should only be seen by the Prolog editor since this will never be true at runtime.

    • current_prolog_flag(dialect, sicstus) is treated as true. Other atomic values than spider and sicstus for the second argument are considered false.

    • In addition, SPIDER handles true, false and some other simple built-ins and control structures. It also handles calls to user-defined, one-clause, zero-argument predicates that only call things SPIDER can handle.

    Note that this restricted language is only used by SPIDER, whereas SICStus itself uses the full Prolog language when evaluating conditional directives.

    The following example shows how conditional compilation can be used to supply SPIDER with special definitions of predicates etc.
    :- if(current_prolog_flag(dialect,spider)).
    %% Dummy definitions for SPIDER. SICStus will not see these.
    :- elif(current_prolog_flag(dialect,sicstus)).
    %% SICStus, but not SPIDER, will see this code
    foo(X,Y) :- some_goal(X,Y).
    %% include some code that is not available in SPIDER
    %% Presumably it defines bar/2.
    :- include(generated_code).
    :- else.
    %% This code is for Prolog dialects other than SICStus.
    foo(X,Y) :- throw(unsupported(foo(X,Y))).
    bar(X,Y) :- throw(unsupported(bar(X,Y))).
    :- endif.

The Indexer

The automatic source code indexer will by default process all files in the project(s). This can lead to unneeded work and irrelevant warnings if there are files that should not be processed, e.g. data files or files written for other dialects of Prolog.

You can select a file, folder, or Project in the Eclipse Project Explorer and select Properties from the context menu. There is a Prolog properties page with the following choices:


If you uncheck it then the Indexer will not process the file or the folder contents except for files loaded by indexed files.

It may be worthwhile to uncheck this for some example folders in the SICStus library tree.

Note that the Indexer will skip non-Prolog files even if Index is checked so you do not need to uncheck this option for non-Prolog files.


Marks a file as the preferred root in a tree of load dependencies. This is useful when the load dependencies form a graph without a unique root. Typically you do not need to explicitly use this as the Indexer will figure out the load tree automatically.


Try to avoid processing the file or folder contents.

It is possible to put these properties in a file located with the affected files. This ensures the properties are set whenever a new project is created and allows version control etc. The details are currently not documented and may change without notice. The curious can deduce the file format from the .spider_data files present below the SICStus Prolog library folder.

There are also some Indexer-related preferences:

Suppress problem reporting for reference projects

default checked. This avoids some work and distractions from computing warnings for the SICStus libraries.

Source Indexing

turn on or off the Indexer completely. Please report if you ever feel the need to turn off the indexer.

Show problems while editing

some problems are expensive to detect. Unchecking this option will postpone expensive problem detection until files are saved.