# Predicate mode indicator \`:\` sometimes means \`+\`

**URL:** <https://swi-prolog.discourse.group/t/predicate-mode-indicator-sometimes-means/4213>\
**Category:** Nice to know\
**Created:** [July 29, 2021, 12:30pm UTC](https://swi-prolog.discourse.group/t/predicate-mode-indicator-sometimes-means/4213 "2021-07-29T12:30:42Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![EricGT](https://avatars.discourse-cdn.com/v4/letter/e/f1d935/32.png) [@EricGT](https://swi-prolog.discourse.group/u/EricGT)\
**Post date:** [July 29, 2021, 12:30pm UTC](https://swi-prolog.discourse.group/t/predicate-mode-indicator-sometimes-means/4213/1 "2021-07-29T12:30:42Z")

</div>

When looking at [predicate modes](https://www.swi-prolog.org/pldoc/man?section=argmode) to understand how to use a predicate one of the first things is to know if the argument is an input (`+`) or output (`-`). However one should also remember that `:` is sometimes `+`.

* * *

The modes for predicates notes

> :  
> Argument is a _meta-argument_ , for example a term that can be called as goal. The predicate is thus a _meta-predicate_ . This flag implies `+` .

* * *

For example in trying to find a predicate that can be run as a query to list all of the known settings when using the [settings library](https://www.swi-prolog.org/pldoc/doc/_SWI_/library/settings.pl),

```prolog
setting/4, % :Name, +Type, +Default, +Comment
setting/2, % :Name, ?Value
set_setting/2, % :Name, +Value
set_setting_default/2, % :Name, +Value
restore_setting/1, % :Name
load_settings/1, % +File
load_settings/2, % +File, +Options
save_settings/0,
save_settings/1, % +File
current_setting/1, % Module:Name
setting_property/2, % ?Setting, ?Property
list_settings/0,
list_settings/1, % +Module
convert_setting_text/3 % +Type, +Text, -Value

```

one would hope to find a predicate with a predicate mode like `+Name, +Value` but there is none.  
If one takes into consideration that `:` is the same as `+` then seeking `:Name, +Value` one finds setting/2. which is actually `:Name, ?Value` but hopefully by now you know that `?` can be considered `+` or `-`.

The mode `:` requires that the pattern of `Module:Name` be used.

Example

```prolog
?- setting(Name,Value).
false.

```

returns false because the pattern `Module:Name` was not used.

```prolog
?- setting(Module:Name,Value).
Module = examples,
Name = setting_01_bool,
Value = true ;
Module = examples,
Name = setting_02_string,
Value = "a string" ;
Module = examples,
Name = setting_03_list,
Value = [1, 2, hello].

```

* * *

In the title I used `sometimes` because one also finds the predicate mode `:Name, +Value` for set\_setting/2 but AFAIK that does not accept unbound values for `:Name`. I don’t know if that is a mistake in the documentation or the fact that `:` does not always mean `+`, something I did wrong in the examples below or something I need to learn.

```plaintext
?- set_setting(Module:Name,Value).
ERROR: Type error: `atom' expected, found `_5016:_5018' (a compound)
ERROR: In:
ERROR: [14] throw(error(type_error(atom,...),_5068))
ERROR: [10] settings:set_setting(_5110:_5112,_5106) at c:/program files/swipl/library/settings.pl:386
ERROR: [9] toplevel_call(user:user: ...) at c:/program files/swipl/boot/toplevel.pl:1116
ERROR: 
ERROR: Note: some frames are missing due to last-call optimization.
ERROR: Re-run your program in debug mode (:- debug.) to get more detail.

?- set_setting(examples:Name,Value).
ERROR: Arguments are not sufficiently instantiated
ERROR: In:
ERROR: [14] throw(error(instantiation_error,_6650))
ERROR: [10] settings:set_setting(examples:_6688,_6682) at c:/program files/swipl/library/settings.pl:386
ERROR: [9] toplevel_call(user:user: ...) at c:/program files/swipl/boot/toplevel.pl:1116
ERROR: 
ERROR: Note: some frames are missing due to last-call optimization.
ERROR: Re-run your program in debug mode (:- debug.) to get more detail.

?- set_setting(examples:setting_01_bool,Value).
ERROR: Arguments are not sufficiently instantiated
ERROR: In:
ERROR: [15] throw(error(instantiation_error,_8296))
ERROR: [10] settings:set_setting(examples:setting_01_bool,_8328) at c:/program files/swipl/library/settings.pl:384
ERROR: [9] toplevel_call(user:user: ...) at c:/program files/swipl/boot/toplevel.pl:1116
ERROR: 
ERROR: Note: some frames are missing due to last-call optimization.
ERROR: Re-run your program in debug mode (:- debug.) to get more detail.

?- set_setting(examples:setting_01_bool,false).
true.

```

* * *

The file used for the examples above:

File: `examples.pl`

```prolog
:- module(examples,[]).

:- use_module(library(settings)).

:- setting(setting_01_bool, boolean, true, 'Example setting of type: boolean.').
:- setting(setting_02_string, text, "a string", 'Example setting of type: string.').
:- setting(setting_03_list, list, [1,2,hello], 'Example setting of type: list.').

```