Templates

"Go is all about type... Type is life." // William Kennedy

Preface

All available data that can be used in YAGPDB's templating "engine" which is slightly modified version of Golang's stdlib text/template package; more in depth and info about actions, pipelines and global functions like printf, index, len,etc > https://golang.org/pkg/text/template/ . This section is meant to be a concise and to the point reference document for all available templates/functions. Functions are covered here. For detailed explanations and syntax guide refer to the learning resource.

Legend: at current state this is still prone to formatting errors, but everything in a code block should refer to a function, parts of a template's action-structure or output returned by YAGPDB; single word/literal-structure in italics refers to type. Methods and fields (e.g. .Append, .User) are usually kept in standard formatting. If argument for a function is optional, it's enclosed in parenthesis ( ). If there are many optional arguments possible, it's usually denoted by 3-dot ...ellipsis.

If functions or methods are denoted with an accent, tilde ~, they are not yet deployed in actual YAGPDB bot or have been disabled in main bot, but are in master code branch.

Always put curly brackets around the data and "actions you perform" you want to formulate as a template like this:{{.User.Username}}

This {{ ... }} syntax of having two curly brackets aka braces around context is necessary to form a template's control structure also known as an action with methods and functions stated below.

Templating system uses standard ASCII quotation marks: 0x22 > " for straight double quotes, 0x27 > 'for apostrophes and 0x60 ` for backticks/back quotes; so make sure no "smart-quotes" are being used.

The difference between back quotes and double quotes in string literals is covered here.

The Dot and Variables

The dot (also known as cursor) {{ . }} encompasses all active data available for use in the templating system, in other words it always refers to current context. For example .User is a Discord User object/structure of current context, meaning the triggering user. To get user object for other users, functions getMember, userArg would help. Same meaning of object/struct applies to other Fields with dot prefix. If it is mentioned as a Method (for example, .Append for type cslice) or as a field on a struct (for example, .User.Bot) then it can not be used alone in template context and always belongs on a parent value. That is, {{.Bot}} would return <no value> whereas {{.User.Bot}} returns bool true/false. Another good example is .Reaction.Emoji.MessageFormat, here you can use .MessageFormat every time you get emoji structure of type discordgo.Emoji, either using reaction triggers or for example .Guild.Emojis. From official docs > "Execution of the template walks the structure and sets the cursor, represented by a period . and called "dot", to the value at the current location in the structure as execution proceeds." All following fields/methods/objects like User/Guild/Member/Channel etc are all part of that dot-structure and there are some more in tables below.

For commenting something inside a template, use this syntax: {{/* this is a comment */}}. May contain newlines. Comments do not nest and they start and end at the delimiters.

To trim spaces use hyphens after/before curyl brackets, for example >{{- /* this is a multi-line comment with whitespace trimmed from preceding and following text */ -}} Using{{- ... -}} is also handy insiderange actions, because whitespaces and newlines are rendered there as output. $ has a special significance in templates, it is set to the starting value of a dot. This means you have access to the global context from anywhere - e.g., inside range/with actions. $ for global context would cease to work if you redefine it inside template, to recover it {{ $ := . }}. $ also denotes the beginning of a variable, which maybe be initialized inside a template action. So data passed around template pipeline can be initialized using syntax > $variable := value. Previously declared variable can also be assigned with new data > $variable = value, it has to have a white-space before it or control panel will error out. Variable scope extends to the end action of the control structure (if, with, range, etc.) in which it is declared, or to the end of custom command if there are no control structures - call it global scope.

Pipes

A powerful component of templates is the ability to stack actions - like function calls, together - chaining one after another. This is done by using pipes |. Borrowed from Unix pipes, the concept is simple: each pipeline’s output becomes the input of the following pipe. One limitation of the pipes is that they can only work with a single value and that value becomes the last parameter of the next pipeline. Example: {{randInt 41 | add 2}} would pipelinerandInt function's return to addition add as second parameter and it would be added to 2; this more simplified would be like {{40 | add 2}} with return 42. If written normally, it would be {{ add 2 (randInt 41) }}. Same pipeline but using a variable is also useful one -{{$x:=40 | add 2}} would not return anything as printout, 40 still goes through pipeline to addition and 42 is stored to variable $x whereas {{($x:=40) | add 2}} would return 42 and store 40 to $x.

Pipes are useful in select cases to shorten code and in some cases improve readability, but they should not be overused. In most cases, pipes are unnecessary and cause a dip in readability that helps nobody.

Context Data

Context data refers to information accessible via the dot, {{ . }}. The accessible data ranges from useful constants to information regarding the environment in which the custom command was executed, such as the user that ran it, the channel it was ran in, and so on.

Fields documented as accessible on specific structures, like the context user .User, are usable on all values that share the same type. That is, given a user $user, $user.ID is a valid construction that yields the ID of the user. Similarly, provided a channel $channel, $channel.Name gives the name of the channel.

Channel

Channel object in Discord documentation.

Channel functions are covered here.

Guild / Server

Guild object in Discord documentation.

Member

Member object in Discord documentation.

Member functions are covered here.

Message

Field

Description

* denotes field that will not have proper return when using getMessage function.

Message object in Discord documentation.

Message functions are covered here.

More information about the Message object can be found here.

Reaction

This is available and part of the dot when reaction trigger type is used.

Reaction object in Discord documentation. Emoji object in Discord documentation.

User

User object in Discord documentation.

User functions are covered here.

Actions

Actions, or elements enclosed in double braces {{ }}, are what makes templates dynamic. Without them, templates would be no more than static text. In this section, we introduce several special kinds of actions which affect the control flow of the program. For example, iteration actions like range and while permit statements to be executed multiple times, while conditional actions like if and with allow for alteration of what statements are ran or are not ran.

If (conditional branching)

Branching using if action's pipeline and comparison operators - these operators don't need to be inside if branch. if statements always need to have an enclosing end. Learning resources covers conditional branching more in depth.

eq , though often used with 2 arguments (eq x y) can actually be used with more than 2. If there are more than 2 arguments, it checks whether the first argument is equal to any one of the following arguments. This behaviour is unique to eq.

Comparison operators always require the same type: i.e comparing 1.23 and 1 would throw incompatible types for comparison error as they are not the same type (one is float, the other int). To fix this, you should convert both to the same type -> for example, toFloat 1.

Range

rangeiterates over element values in variety of data structures in pipeline - integers, slices/arrays, maps or channels. The dot . is set to successive elements of those data structures and output will follow execution. If the value of pipeline has zero length, nothing is output or if an {{else}} action is used, that section will be executed.

To skip execution of a single iteration and jump to the next iteration, the {{continue}} action may be used. Likewise, if one wishes to skip all remaining iterations, the {{break}} action may be used. These both are usable also inside while action.

Affected dot inside range is important because methods mentioned above in this documentation:.Server.ID, .Message.Content etc are all already using the dot on the pipeline and if they are not carried over to the range control structure directly, these fields do not exists and template will error out. Getting those values inside range and also with action would need $.User.ID for example. range on slices/arrays provides both the index and element for each entry; range on map iterates over key/element pairs. If a range action initializes a variable, that variable is set to the successive elements of the iteration. range can also declare two variables, separated by a comma and set by index and element or key and element pair. In case of only one variable, it is assigned the element. Like if, rangeis concluded with{{end}}action and declared variable scope inside range extends to that point.

{{/* range over an integer */}}
{{range 2}}{{.}}{{end}}
{{range $k, $v := toInt64 2}}{{$k}}{{$v}}{{end}}
{{/* range over a slice */}}
{{ range $index, $element := cslice "YAGPDB" "IS COOL!" }}
{{ $index }} : {{ $element }} {{ end }}
{{/* range on a map */}}
{{ range $key, $value := dict "SO" "SAY" "WE" "ALL!" }}
{{ $key }} : {{ $value }} {{ end }}
{{/* range with else and variable scope */}}
{{ range seq 1 1 }} no output {{ else }} output here {{ end }}
{{ $x := 42 }} {{ range $x := seq 2 4 }} {{ $x }} {{ end }} {{ $x }}

Custom command response was longer than 2k (contact an admin on the server...) or Failed executing template: response grew too big (>25k)

This is quite common error users will get whilst using range. Simple example to reproduce it: {{ range seq 0 10000 }} {{ $x := . }} {{ end }} HELLO! This will happen because of whitespaces and newlines, so make sure you one-line the range or trim spaces, in this context {{- $x := . -}}

Try-catch

Multiple template functions have the possibility of returning an error upon failure. For example, dbSet can return a short write error if the size of the database entry exceeds some threshold.

While it is possible to write code that simply ignores the possibility of such issues occuring (letting the error stop the code completely), there are times at which one may wish to write more robust code that handles such errors gracefully. The try-catch construct enables this possibility.

Similar to an if action with an associated else branch, the try-catch construct is composed of two blocks: the try branch and the catch branch. First, the code in the try branch is ran, and if an error is raised by a function during execution, the catch branch is executed instead with the context (.) set to the offending error.

To check for a specific error, one can compare the result of the Error method with a predetermined message. (For context, all errors have a method Error which is specified to return a message describing the reason that the error was thrown.) For example, the following example has different behavior depending on whether "Reaction blocked" is in the message of the error caught.

{{ try }}
    {{ addReactions "👋" }}
    added reactions successfully
{{ catch }}
    {{ if in .Error "Reaction blocked" }}
        user blocked YAG :(
    {{ else }}
        different issue occurred: {{ .Error }}
    {{ end }}
{{ end }}

While

while iterates as long as the specified condition is true, or more generally evaluates to a non-empty value. The dot (.) is not affected, unlike with the range action. Analogous to range, while introduces a new scope which is concluded by the end action. Within the body of a while action, the break and continue actions can be used to appropriate effect, like in a range action.

{{/* efficiently search for an element in a sorted slice using binary search */}}
{{ $xs := cslice 1 3 5 6 6 8 10 12 }}
{{ $needle := 8 }}

{{ $lo := 0 }}
{{ $hi := sub (len $xs) 1 }}
{{ $found := false }}
{{/* it's possible to combine multiple conditions using logical operators */}}
{{ while and (le $lo $hi) (not $found) }}
	{{- $mid := div (add $lo $hi) 2 }}
	{{- $elem := index $xs $mid }}
	{{- if lt $elem $needle }}
		{{- $lo = add $mid 1 }}
	{{- else if eq $elem $needle }}
		{{- print "found at index " $mid }}
		{{- $found = true }}
	{{- else }}
		{{- $hi = sub $mid 1 }}
	{{- end -}}
{{ end }}
{{ if not $found }} not found {{ end }}

With

with lets you assign and carry pipeline value with its type as a dot (.) inside that control structure, it's like a shorthand. If the value of the pipeline is empty, dot is unaffected and when an else or else if action is used, execution moves on to those branches instead, similar to the if action. Affected dot inside with is important because methods mentioned above in this documentation:.Server.ID, .Message.Content etc are all already using the dot on the pipeline and if they are not carried over to the with control structure directly, these fields do not exists and template will error out. Getting those values inside with and also range action would need $.User.ID for example.

Like if and range actions, with is concluded using {{end}} and variable scope extends to that point.

{{/* Shows the scope and how dot is affected by object's value in pipeline */}}
{{ $x := "42" }} {{ with and ($z:= seq 0 5) ($x := seq 0 10) }} 
len $x: `{{ len $x }}` 
{{/* "and" function uses $x as last value for dot */}}
same as len dot: `{{ len . }}` 
but len $z is `{{ len $z }}` {{ end }}
Outer-scope $x len however: {{ len $x }}
{{/* when there's no value, dot is unaffected */}}
{{ with false }} dot is unaffected {{ else }} printing here {{ .CCID }} {{ end }}
{{/* using else-if chain is possible */}}
{{ with false }}
    not executed
{{ else if eq $x "42" }}
    x is 42, dot is unaffected {{ .User.Mention }}
{{ else if eq $x "43" }}
    x is not 43, so this is not executed
{{ else }}
    branch above already executed, so else branch is not
{{ end }}

Associated Templates

Templates (i.e., custom command programs) may also define additional helper templates that may be invoked from the main template. Technically speaking, these helper templates are referred to as associated templates. Associated templates can be used to create reusable procedures accepting parameters and outputting values, similar to functions in other programming languages.

Definition

To define an associated template, use the define action. It has the following syntax:

{{ define "template_name" }}
    {{/* associated template body */}}
{{ end }}

Warning: Template definitions must be at the top level of the custom command program; in other words, they cannot be nested in other actions (for example, an if action.) That is, the following custom command is invalid:

{{ if $cond }}
    {{ define "hi" }} hi! {{ end }}
{{ end }}

The template name can be any string constant; however, it cannot be a variable, even if said variable references a value of string type. As for the body of the associated template body, it can be anything that is a standalone, syntactically valid template program. Note that the first criterion precludes using variables defined outside of the associated template; that is, the following custom command is invalid, as the body of the associated template references a variable ($name) defined in an outer scope:

{{ $name := "YAG" }}
{{ define "hello" }}
    Hello, {{ $name }}!
{{ end }}

If accessing the value of $name is desired, then it needs to be passed as part of the context when executing the associated template.

Within the body of an associated template, the variable $ and the context dot (.) both initially refer to the data passed as context during execution. Consequently, any data on the original context that needs to be accessed must be explicitly provided as part of the context data. For example, if one wishes to access .User.Username in an associated template body, it is necessary to pass .User.Username as part of the context data when executing said template.

To return a value from an associated template, use the return action. Encountering a return action will cause execution of the associated template to end immediately and control to be returned to the caller. For example, below is an associated template that always returns 1:

{{ define "getOne" }} {{ return 1 }} {{ end }}

Note that it is not necessary for a value to be returned; {{ return }} by itself is completely valid.

Note: Since all custom commands are themselves templates, using a return action at the top level is perfectly valid, and will result in execution of the custom command being stopped at the point the return is encountered.

{{ if not .CmdArgs }}
    no arguments passed
    {{ return }} {{/* anything beyond this point is not executed */}}
{{ end }}
{{ $firstArg := index .CmdArgs 0 }}
{{/* safe since .CmdArgs is guaranteed to be non-empty here */}}

Execution

To execute a custom command, one of three methods may be used: template, block, or execTemplate.

Template action

template is a function-like action that executes the associated template with the name provided, ignoring its return value. Note that the name of the template to execute must be a string constant; similar to define actions, a variable referencing a value of string type is invalid. Data to use as the context may optionally be provided following the name.

While template is function-like, it is not an actual function, leading to certain quirks; notably, it must be used alone, not part of another action (like a variable declaration), and the data argument need not be parenthesized. Due to this, it is recommended that execTemplate, which has much more intuitive behavior, be used instead of the template action if at possible.

Below is an example of the template action in action:

{{ define "sayHi" }}
    {{- if . -}}
        hi there, {{ . }}
    {{- else }}
        hi there!
    {{- end -}}
{{ end }}
{{ template "sayHi" }} {{/* hi there! */}}
{{ template "sayHi" "YAG" }} {{/* hi there, YAG */}}

Trim markers: {{- ... -}}were used in above example because whitespace is considered as part of output for associated template definitions (and actions in general).

Block action

block has a structure similar to that of a define action. It is equivalent to a define action followed by a template action:

{{ $name := "YAG" }}
{{ block "sayHi" $name }}
    hi there, {{ . }}
{{ end }}

{{/* equivalent to above */}}
{{ define "sayHi" }}
    hi there, {{ . }}
{{ end }}
{{ template "sayHi" $name }}

execTemplate function

execTemplate is essentially the same as the template action, but it provides access to the return value of the template and may be used as part of another action. Below is an example using execTemplate:

{{ define "factorial" }}
    {{- $n := 1 }}
    {{- range seq 2 (add . 1) }}
        {{- $n = mult $n . }}
    {{- end }}
    {{- return $n -}}
{{ end }}

{{ $fac := execTemplate "factorial" 5 }}
2 * 5! = {{ mult $fac 2 }}

Custom Types

Golang has built-in primitive data types (int, string, bool, float64, ...) and built-in composite data types (array, slice, map, ...) which also are used in custom commands. YAGPDB's templating "engine" has currently two user-defined, custom data types - templates.Slice and templates.SDict. There are other custom data types used like discordgo.Timestamp, but these are outside of the main code of YAGPDB, so not explained here further. Type time.Time is covered in its own section. Custom Types section discusses functions that initialize values carrying those templates.Slice (abridged to cslice), templates.SDict (abridged to sdict) types and their methods. Both types handle type interface{} element. It's called an empty interface which allows a value to be of any type. So any argument of any type given is handled. (In "custom commands"-wise mainly primitive data types, but slices as well.)

Reference type-like behaviour: Slices and dictionaries in CCs exhibit reference-type like behavior, which may be undesirable in certain situations. That is, if you have a variable $x that holds a slice/dictionary, writing $y := $x and then mutating $y via Append/Set/Del/etc. will modify $x as well. For example:

{{ $x := sdict "k" "v" }}
{{ $y := $x }}
{{ $y.Set "k" "v2" }} {{/* modify $y */}}
{{ $x }}
{{/* k has value v2 on $x as well - 
that is, modifying $y changed $x too. */}}

If this behaviour is undesirable, copy the slice/dictionary via cslice.AppendSlice or a range + Set call .

{{ $x := sdict "k" "v" }}
{{ $y := sdict }}
{{ range $k, $v := $x }} {{- $y.Set $k $v -}} {{ end }}
{{ $y.Set "k" "v2" }}
{{ $x }} {{/* $x is unmodified - k still has value v */}}

Note that this performs a shallow copy, not a deep copy - if you want the latter you will need to perform the aforementioned operation recursively.

templates.Slice

templates.Slice - This is a custom composite data type defined using an underlying data type []interface{} . It is of kind slice (similar to array) having interface{} type as its value and can be initialized using cslice function. Retrieving specific element inside templates.Slice is by indexing its position number.

This section's snippets:

To demonstrate .StringSlice {{(cslice currentTime.Month 42 "YAPGDB").StringSlice}} will return a slice [February YAGPDB]. If the flag would have been set to true - {{...).StringSlice true}}, all elements in that slice were not strings and <no value> is returned.

General example:

Creating a new cslice: {{ $x := (cslice "red" "red") }} **{{ $x }}**
Appending to current cslice data 
and assigning newly created cslice to same variable:
{{ $x = $x.Append "green" }} **{{ $x }}**
Setting current cslice value in position 1:
{{ $x.Set 1 "blue" }} **{{ $x }}**
Indexing that position 1:
**{{ index $x 1 }}**
Appending a slice to current cslice data 
but not assigning newly created cslice to same variable:
**{{ $x.AppendSlice (cslice "yellow" "magenta") }}**
Variable is still: **{{ $x }}**
Type of variable: **{{ printf "%T" $x }}**

templates.SDict

templates.SDict - This is a custom composite data type defined on an underlying data type map[string]interface{}. This is of kind map having string type as its key and interface{} type as that key's value and can be initialized using sdict function. A map is key-value store. This means you store value and you access that value by a key. Map is an unordered list and the number of parameters to form key-value pairs must be even, difference to regular map is that templates.SDict is ordered by its key. Retrieving specific element inside templates.Sdict is by indexing its key.

Creating sdict: {{ $x := sdict "color1" "green" "color2" "red" }} **{{ $x }}**
Retrieving key "color2": **{{ $x.Get "color2" }}**
Changing "color2" to "yellow": {{ $x.Set "color2" "yellow" }} **{{ $x }}**
Adding "color3" as "blue": {{ $x.Set "color3" "blue" }} **{{ $x }}**
Deleting key "color1" {{ $x.Del "color1" }} and whole sdict: **{{ $x }}**

Tip: Previously, when saving cslices, sdicts, and dicts into database, they were serialized into their underlying native types - slices and maps. This meant that if you wanted to get the custom type back, you needed to convert manually, e.g. {{cslice.AppendSlice $dbSlice}} or {{sdict $dbDict}}. Recent changes to YAG have changed this: values with custom types are now serialized properly, making manual conversion unnecessary.

Database

You have access to a basic set of Database functions having return of type *customcommands.LightDBEntry called here DBEntry. This is almost a key value store ordered by the key and value combined.

You can have max 50 * user_count (or 500 * user_count for premium) values in the database, if you go above this all new write functions will fail, this value is also cached so it won't be detected immediately when you go above nor immediately when you're under again.

Patterns are basic PostgreSQL patterns, not Regexp: An underscore (_) matches any single character; a percent sign (%) matches any sequence of zero or more characters.

Keys can be max 256 bytes long and has to be strings or numbers. Values can be anything, but if their serialized representation exceeds 100kB a short write error gets raised.

You can just pass a userIDof 0 to make it global (or any other number, but 0 is safe). There can be 10 database interactions per CC, out of which dbTop/BottomEntries, dbCount, dbGetPattern, and dbDelMultiple may only be run twice. (50,10 for premium users).

Learning resources covers database more in-depth.

Database functions are covered here.

Example here.

DBEntry

Tickets

Ticket functions are limited to 1 call per custom command for both normal and premium guilds and limited to max 10 open tickets per user.

Template Ticket

Time

Time and duration types use Golang's time package library and its methods > https://golang.org/pkg/time/#time and also this although slightly different syntax all applies here > https://gobyexample.com/time.

Time functions are covered here.

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