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This module provides a consistent interface for processing boolean or boolean-style string input. While Lua allows the <code>true</code> and <code>false</code> boolean values, wikicode templates can only express boolean values through strings such as "yes", "no", etc. This module processes these kinds of strings and turns them into boolean input for Lua to process. It also returns <code>nil</code> values as <code>nil</code>, to allow for distinctions between <code>nil</code> and <code>false</code>. The module also accepts other Lua structures as input, i.e. booleans, numbers, tables, and functions. If it is passed input that it does not recognise as boolean or <code>nil</code>, it is possible to specify a default value to return.

== Syntax ==

<syntaxhighlight lang="lua">yesno(value, default)</syntaxhighlight>

<code>value</code> is the value to be tested. Boolean input or boolean-style input (see below) always evaluates to either <code>true</code> or <code>false</code>, and <code>nil</code> always evaluates to <code>nil</code>. Other values evaluate to <code>default</code>.

== Usage ==

First, load the module. Note that it can only be loaded from other Lua modules, not from normal wiki pages. For normal wiki pages you can use {{tl|yesno}} instead.

<syntaxhighlight lang="lua">
local yesno = require('Module:Yesno')
</syntaxhighlight>

Some input values always return <code>true</code>, and some always return <code>false</code>. <code>nil</code> values always return <code>nil</code>.

<syntaxhighlight lang="lua">
-- These always return true:
yesno('yes')
yesno('y')
yesno('true')
yesno('t')
yesno('1')
yesno(1)
yesno(true)

-- These always return false:
yesno('no')
yesno('n')
yesno('false')
yesno('f')
yesno('0')
yesno(0)
yesno(false)

-- A nil value always returns nil:
yesno(nil)
</syntaxhighlight>

String values are converted to lower case before they are matched:

<syntaxhighlight lang="lua">
-- These always return true:
yesno('Yes')
yesno('YES')
yesno('yEs')
yesno('Y')
yesno('tRuE')

-- These always return false:
yesno('No')
yesno('NO')
yesno('nO')
yesno('N')
yesno('fALsE')
</syntaxhighlight>

You can specify a default value if yesno receives input other than that listed above. If you don't supply a default, the module will return <code>nil</code> for these inputs.

<syntaxhighlight lang="lua">
-- These return nil:
yesno('foo')
yesno({})
yesno(5)
yesno(function() return 'This is a function.' end)

-- These return true:
yesno('foo', true)
yesno({}, true)
yesno(5, true)
yesno(function() return 'This is a function.' end, true)

-- These return "bar":
yesno('foo', 'bar')
yesno({}, 'bar')
yesno(5, 'bar')
yesno(function() return 'This is a function.' end, 'bar')
</syntaxhighlight>

Note that the blank string also functions this way:
<syntaxhighlight lang="lua">
yesno('')        -- Returns nil.
yesno('', true)  -- Returns true.
yesno('', 'bar') -- Returns "bar".
</syntaxhighlight>

Although the blank string usually evaluates to false in wikitext, it evaluates to true in Lua. This module prefers the Lua behaviour over the wikitext behaviour. If treating the blank string as false is important for your module, you will need to remove blank arguments at an earlier stage of processing.<includeonly>{{#ifeq:{{SUBPAGENAME}}|sandbox||
[[Category:Lua metamodules]]
}}</includeonly>