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Power Fx YAML formula grammar

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Microsoft Power Fx is the new name for the formula language for canvas apps. These articles are a work in progress as we extract the language from canvas apps, integrate it with other Microsoft Power Platform products, and make it available as open source. Start with the Microsoft Power Fx overview for an introduction to the language.

Microsoft Power Fx has a well-established grammar for expressions based on Excel. However, when used in Power Apps and other hosts where UI provides the name-to-expression binding for a formula, there is no standard way of editing the formula binding as text.

We've selected the industry standard YAML as our language for this binding. There are already a large number of editors, tools, and libraries for working with YAML. This article describes how we represent formulas in YAML.

At this time, we support only a restricted subset of YAML. Only the constructs described in this article are supported.

Not everything that defines a canvas app is represented here; additional information flows through other files that the tool produces and consumes.

Leading equal sign

First and foremost, all expressions must begin with a leading equal sign =:

Visible: =true
X: =34
Text: |
	="Hello, " &
	"World"

We use the = in this manner for three reasons:

  • It's consistent with Excel, which uses a leading = to bind an expression to a cell.
  • It effectively escapes the formula language's syntax so that YAML doesn't attempt to parse it. Normally, YAML would treat text: 1:00 as minutes and seconds, converting it to a number. By inserting an =, YAML won't use its implicit typing rules and formulas won't be harmed. Using = covers most cases, but not all, and those exceptions are described in the following section, Single-line formulas.
  • In the future, we will support both formulas (starts with =) and non-formulas (no =) in the same file, just as Excel does We can do this in YAML and non-YAML files alike across Microsoft Power Platform source files. Anywhere a formula is supported, the leading = differentiates a Power Apps formula expression from a static scalar value.

Single-line formulas

Single-line formulas are written in the form:

Name : SPACE = Expression

The space between the colon and the equal sign is required to be YAML-compliant. The equal sign disrupts YAML's normal interpretation of the expression, allowing the rest of the line to be interpreted as Power Fx. For example:

Text1: ="Hello, World"
Text2: ="Hello " & ", " & "World"
Number1: =34
Boolean1: =true
Time1: =1:34

The number sign # and colon : aren't allowed anywhere in single-line formulas, even if they're in a quoted text string or identifier name. To use a number sign or colon, you must express the formula as a multiline formula. The number sign is interpreted as a comment in YAML, and the colon is interpreted as a new name map in YAML. To add a comment to a single-line comment, use the Power Fx line comment starting with //.

Using normal YAML escaping with single quotes and C-like backslashes isn't supported; use a multiline formula instead. This is for consistency and to facilitate cut/paste between the formula bar in Power Apps Studio and YAML source files.

See the canvas apps operators and identifiers documentation for details on allowed names and the structure of an expression.

Multiline formulas

Formulas can span multiple lines by using YAML's block scalar indicators:

Name : SPACE ( | or |+ or |- )  = Expression-Line  Expression-Line  ...

All lines that are a part of the block must be indented at least one space in from the level of the first line.

For example:

Text1: |
    ="Hello, World"
Text2: |
    ="Hello" &
    "," &
    "World"

All forms of YAML multiline scalar notations are accepted on import, including >+, for example. However, to ensure that whitespace is properly preserved, only |, |+, or |- are produced.

Component instance

Components are instanced by using YAML object notation. The type of the object is established with the As operator as a part of the left-side YAML tag. For container controls, objects can be nested.

Name As Component-Type [ . Component-Template ] :  ( Single-Line-Formula or Multi-Line-Formula or Object-instance )  ...

All lines that are a part of the block must be indented at least one space in from the level of the first line.

For example:

Gallery1 As Gallery.horizontalGallery:
    Fill: = Color.White
    Label1 As Label:
        Text: ="Hello, World"
        X: =20
        Y: =40
        Fill: |
            =If( Lower( Left( Self.Text, 6 ) ) = "error:",
                Color.Red,
                Color.Black
            )

Component-Type can be any canvas component or control. Base types, such as Number, aren't supported.

Component-Template is an optional specifier for components that have different templates, such as the Gallery. Not all components have templates.

If Name contains special characters and is wrapped with single quotation marks, the entire phrase to the left side of the colon will need to be escaped. This can be done in one of the following ways:

  • Use single quotation marks to wrap the entire left side, which requires that the existing single quotation marks be used twice: 
    '''A name with a space'' As Gallery':
  • Use double quotation marks to wrap the entire left side, but be sure that there are no double quotation marks in the name: 
    "'A name with a space' As Gallery":

Component definition

Similarly, components are defined by creating an instance of one of the supported base types. The base types can't be instanced directly. Within an object definition, properties can be added to what the base type provides.

The supported base types are: CanvasComponent

Simple property definition

Components use properties to communicate with the app in which they're hosted.

Name : ( Single-Line-Expression or Multi-Line-Expression )

The type of the formula is implied by the type of the expression.

For input properties, the expression provides the default to be inserted into the app when the component is instanced. The maker can modify this expression as they see fit, but can't change the type.

For output properties, the expression provides the calculation to be performed. The maker can't modify this expression, it's encapsulated in the component.

At this time, all properties are data flow only and can't contain side effects.

At this time, additional metadata about the property isn't defined here but is instead defined in the other files of the .msapp file, for example the property's description.

For example:

DateRangePicker As CanvasComponent:
    DefaultStart: |-
		=// input property, customizable default for the component instance
		Now()                      
    DefaultEnd: |-
		=// input property, customizable default for the component instance
		DateAdd( Now(), 1, Days )    
    SelectedStart: =DatePicker1.SelectedDate   // output property
    SelectedEnd: =DatePicker2.SelectedDate     // output property

YAML compatibility

YAML comments

YAML line comments delimited by the number sign # aren't preserved anywhere in the source format. Instead, within a formula, delimit line comments with // characters or block comments with /* and */. More information: Comments

Errors for common pitfalls

There are a few places where the Power Fx and YAML grammars are incompatible or might be confusing for a user. In these cases, an error is thrown.

For example, in the following:

Text: ="Hello #PowerApps"
Record: ={ a: 1, b: 2 }

the number sign # is treated as a comment by YAML, even though it's embedded in what Excel considers a text string (wrapped by double quotation marks). To avoid confusion, this case will throw an error during import. A YAML multiline form can be used instead.

In the case of the value for record, YAML considers a: and b: to be another name map binding. YAML allows the same name map to be reused, with the last one silently overriding any previous definitions. Because this can be confusing for a low-code maker and can result in the loss of a property formula, an error is thrown if the same name is encountered twice.