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Polishing up boolean logic and types

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Adam Hollett
2015-10-16 10:53:23 -04:00
parent f2a1925033
commit 0147832a64
2 changed files with 57 additions and 101 deletions
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_basics/truthy-and-falsy.md
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title: Truthy and Falsy
---
In programming, we describe “truthy” and “falsy” as anything that returns true or false, respectively, when used inside an if statement.
In programming, anything that returns `true` in a conditional is called **truthy**. Anything that returns `false` in a conditional is called **falsy**. All object types can be described as either truthy or falsy.
## What is truthy?
## Truthy
All values in Liquid are truthy, with the exception of `nil` and `false`.
All values in Liquid are truthy except `nil` and `false`.
In the example below, the text “Tobi” is not a boolean, but it is truthy in a conditional:
In the example below, the text "Tobi" is not a boolean, but it is truthy in a conditional:
{% highlight html %}{% raw %}
{% highlight liquid %}{% raw %}
{% assign tobi = 'Tobi' %}
{% if tobi %}
This will always be true.
{% if tobi == true %}
This condition will always be true.
{% endif %}
{% endraw %}{% endhighlight %}
[Strings](/themes/liquid-documentation/basics/types/#strings), even when empty, are truthy. The example below will result in empty HTML tags if `settings.fp_heading` is empty:
[Strings](/basics/types/#string), even when empty, are truthy. The example below will result in empty HTML tags if `settings.fp_heading` is empty:
<p class="input">Input</p>
{% highlight html %}{% raw %}
{% highlight liquid %}{% raw %}
{% if settings.fp_heading %}
<h1>{{ settings.fp_heading }}</h1>
<h1>{{ settings.fp_heading }}</h1>
{% endif %}
{% endraw %}{% endhighlight %}
<p class="output">Output</p>
{% highlight html %}{% raw %}
<h1></h1>
{% endraw %}{% endhighlight %}
To avoid this, you can check to see if the string is <code>blank</code>, as follows:
<div>
{% highlight html %}{% raw %}
{% unless settings.fp_heading == blank %}
<h1>{{ settings.fp_heading }}</h1>
{% endunless %}
{% endraw %}{% endhighlight %}
</div>
<hr/>
An [EmptyDrop](/themes/liquid-documentation/basics/types/#empty-drop) is also truthy. In the example below, if `settings.page` is an empty string or set to a hidden or deleted object, you will end up with an EmptyDrop. The result is an undesirable empty &lt;div&gt;:
[EmptyDrops](/basics/types/#emptydrop) are also truthy. In the example below, if `settings.page` is an empty string or set to a hidden or deleted object, you will end up with an EmptyDrop. The result is an empty `<div>`:
<p class="input">Input</p>
{% highlight html %}{% raw %}
{% if pages[settings.page] %}
<div>{{ pages[settings.page].content }}</div>
<div>{{ pages[settings.page].content }}</div>
{% endif %}
{% endraw %}{% endhighlight %}
<p class="output">Output</p>
{% highlight html %}{% raw %}
<div></div>
{% endraw %}{% endhighlight %}
## Falsy
## What is falsy?
The only values that are falsy in Liquid are `nil` and `false`.
[nil](/themes/liquid-documentation/basics/types/#nil) is returned when a Liquid object doesn't have anything to return. For example, if a collection doesn't have a collection image, collection.image will be set to <tt>nil</tt>. Since that is “falsy”, you can do this:
{% highlight html %}{% raw %}
{% if collection.image %}
<!-- output collection image -->
{% endif %}
{% endraw %}{% endhighlight %}
The value <tt>false</tt> is returned through many Liquid object properties such as <tt>product.available</tt>.
The falsy values in Liquid are [nil](/basics/types/#nil) and [false](/basics/types/#boolean).
## Summary
@@ -80,16 +56,15 @@ The table below summarizes what is truthy or falsy in Liquid.
| | truthy | falsy |
| ------------- |:-------------:|:-------------:|
| true | &times; | |
| false | | &times; |
| nil | | &times; |
| string | &times; | |
| empty string | &times; | |
| 0 | &times; | |
| 1 or 2 or 3.14 | &times; | |
| array | &times; | |
| empty array | &times; | |
| collection | &times; | |
| collection with no products | &times; | |
| page | &times; | |
| EmptyDrop | &times; | |
| true | • | |
| false | | • |
| nil | | • |
| string | • | |
| empty string | • | |
| 0 | • | |
| integer | • | |
| float | • | |
| array | • | |
| empty array | • | |
| page | • | |
| EmptyDrop | • | |
_basics/types.md
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@@ -2,14 +2,14 @@
title: Types
---
Liquid objects can return one of six types:
Liquid objects can have one of six types:
- [string](#string)
- [number](#number)
- boolean
- nil
- array
- EmptyDrop
- [boolean](#boolean)
- [nil](#nil)
- [array](#array)
- [EmptyDrop](#emptydrop)
Liquid variables can be initialized by using the [assign](/tags/#assign) or [capture](/tags/#capture) tags.
@@ -34,7 +34,7 @@ Numbers include floats and integers.
{% endraw %}
{% endhighlight %}
### Booleans
### Boolean
Booleans are either `true` or `false`. No quotations are necessary when declaring a boolean.
@@ -65,26 +65,26 @@ Tags or outputs that return `nil` will not print anything to the page.
<p class="input">Input</p>
{% highlight html %}{% raw %}
{% highlight liquid %}{% raw %}
The current user is {{ user.name }}
{% endraw %}{% endhighlight %}
<p class="output">Output</p>
{% highlight html %}{% raw %}
{% highlight text %}{% raw %}
The current user is
{% endraw %}{% endhighlight %}
### Arrays
### Array
Arrays hold lists of variables of any type.
#### Accessing items in arrays
To access items in an array, you can loop through each item in the array using a [for](/tags/#for) or [tablerow](/tags/#tablerow) tag.
To access all of the items in an array, you can loop through each item in the array using a [for](/tags/#for) or [tablerow](/tags/#tablerow) tag.
<p class="input">Input</p>
{% highlight html %}{% raw %}
{% highlight liquid %}{% raw %}
<!-- if site.users = "Tobi", "Lina", "Tetsuro", "Adam" -->
{% for user in site.users %}
{{ user }}
@@ -92,17 +92,17 @@ To access items in an array, you can loop through each item in the array using a
{% endraw %}{% endhighlight %}
<p class="output">Output</p>
{% highlight html %}{% raw %}
{% highlight text %}{% raw %}
Tobi Lina Tetsuro Adam
{% endraw %}{% endhighlight %}
#### Accessing a specific item in an array
#### Accessing specific items in arrays
You can use square bracket `[ ]` notation to access a specific item in an array. Array indexing starts at zero.
<p class="input">Input</p>
{% highlight html %}{% raw %}
{% highlight liquid %}{% raw %}
<!-- if site.users = "Tobi", "Lina", "Tetsuro", "Adam" -->
{{ site.users[0] }}
{{ site.users[1] }}
@@ -110,67 +110,48 @@ You can use square bracket `[ ]` notation to access a specific item in an array.
{% endraw %}{% endhighlight %}
<p class="output">Output</p>
{% highlight html %}{% raw %}
{% highlight text %}{% raw %}
Tobi
Lina
Adam
{% endraw %}{% endhighlight %}
#### Initializing an array
#### Initializing arrays
It is not possible to initialize an array using only Liquid.
You cannot initialize arrays using pure Liquid.
You can, howver, use the [split](/filters/#split) filter to break a single string into an array of substrings.
You can, however, use the [split](/filters/#split) filter to break a single string into an array of substrings.
## EmptyDrop
An EmptyDrop object is returned whenever you try to access a non-existent object (for example, a collection, page or blog that was deleted or hidden) by [handle](/basics/#Handles). In the example below, `page_1`, `page_2` and `page_3` are all EmptyDrop objects.
An EmptyDrop object is returned if you try to access a deleted object (such as a page or post) by its [handle](/basics/#Handles). In the example below, `page_1`, `page_2` and `page_3` are all EmptyDrop objects.
{% highlight html %}{% raw %}
{% highlight liquid %}{% raw %}
{% assign variable = "hello" %}
{% assign page_1 = pages[variable] %}
{% assign page_2 = pages["i-do-not-exist-in-your-store"] %}
{% assign page_3 = pages.this-handle-does-not-belong-to-any-page %}
{% assign page_2 = pages["does-not-exist"] %}
{% assign page_3 = pages.this-handle-does-not-exist %}
{% endraw %}{% endhighlight %}
EmptyDrop objects only have one attribute, <code>empty?</code>, which is always true.
EmptyDrop objects only have one attribute, `empty?`, which is always *true*.
Collections and pages that _do_ exist do not have an <code>empty?</code> attribute. Their <code>empty?</code> is “falsy”, which means that calling it inside an if statement will return <tt>false</tt>. When using an unless statement on existing collections and pages, <code>empty?</code> will return <tt>true</tt>.
Collections and pages that *do* exist do not have an `empty?` attribute. Their `empty?` is “falsy”, which means that calling it inside an if statement will return *false*. When using an unless statement on existing collections and pages, `empty?` will return `true`.
#### Applications in themes
#### Checking for emptiness
Using the <code>empty?</code> attribute, you can check to see if a page exists or not _before_ accessing any of its other attributes.
Using the `empty?` attribute, you can check to see if an object exists or not before you access any of its attributes.
{% highlight html %}{% raw %}
{% unless pages.frontpage.empty? %}
<!-- We have a page with handle 'frontpage' and it's not hidden.-->
{% highlight liquid %}{% raw %}
{% unless pages.about.empty? %}
<!-- This content will only print if the page with handle 'about' is not empty -->
<h1>{{ pages.frontpage.title }}</h1>
<div>{{ pages.frontpage.content }}</div>
{% endunless %}
{% endraw %}{% endhighlight %}
It is important to see if a page exists or not first to avoid outputting empty HTML elements to the page, as follows:
If you don't check for emptiness first, Liquid may print empty HTML elements to the page:
{% highlight html %}{% raw %}
<h1></h1>
<div></div>
{% endraw %}{% endhighlight %}
You can perform the same verification with collections as well:
{% highlight html %}{% raw %}
{% unless collections.frontpage.empty? %}
{% for product in collections.frontpage.products %}
{% include 'product-grid-item' %}
{% else %}
<p>We do have a 'frontpage' collection but it's empty.</p>
{% endfor %}
{% endunless %}
{% endraw %}{% endhighlight %}