03-using-expressions.md

Using Bicep expressions

In the first tutorial, we declared a basic storage account resource and augmented that declaration with references to declared parameters and variables. These references are forms of expressions.

Even though the bicep language is still in its infancy, you can already write some powerful expressions to take advantage of in your bicep files.

Using a bicep function

Any valid ARM Template function is also a valid bicep function. Resource functions do not yet understand new concepts like the symbolic resource name, so they still require a resourceId.

The following are all valid function calls in bicep:

param serverFarmId string = resourceId('Microsoft.Web/sites', 'myWebsite')

var location = resourceGroup().location

output makeCapital string = toUpper('all lowercase')

In our main.bicep file, instead of forcing users to guess a unique storage account name, let's get rid of our name parameter and use the uniqueString() and resourceGroup() functions to calculate a unique name. We'll also use the resourceGroup().location property instead of hardcoding a default location.

param location string = resourceGroup().location

var storageSku = 'Standard_LRS' // declare variable and assign value

resource stg 'Microsoft.Storage/storageAccounts@2019-06-01' = {
    name: uniqueString(resourceGroup().id) // generates unique name based on resource group ID
    location: location
    kind: 'Storage'
    sku: {
        name: storageSku // assign variable
    }
}

output storageId string = stg.id

Using string interpolation

The concat() function is one of the most commonly used ARM Template functions and can add a lot of verbosity to a template. To simplify this, we now support a string interpolation syntax.

For example, I could combine a namePrefix parameter with a hardcoded suffix:

param namePrefix string = 'unique'

var storageAccountName = '${namePrefix}storage001'

Which is equivalent to the following ARM Template concat() function:

{
  "variables": {
    "storageAccountName": "[concat(parameters('namePrefix'), 'storage001')]"
  }
}

Let's use a namePrefix parameter and concatenate that with our uniqueString() using string interpolation. We'll also use the variable storageAccountName to store this expression, so that our resource declaration is a bit cleaner:

param location string = resourceGroup().location
param namePrefix string = 'stg'

var storageSku = 'Standard_LRS' // declare variable and assign value
var storageAccountName = '${namePrefix}${uniqueString(resourceGroup().id)}'

resource stg 'Microsoft.Storage/storageAccounts@2019-06-01' = {
    name: storageAccountName
    location: location
    kind: 'Storage'
    sku: {
        name: storageSku
    }
}

output storageId string = stg.id

If you compile with bicep build, you will notice we are compiling this into the format() function, not the concat() function. Functionally, the end result is the same, but the format() function is a bit more stable in the IL.

Using the ternary operator

You can conditionally provide a value for a variable, resource, or output using the ternary operator, which is the equivalent of the if() function in ARM Templates. Instead of using a variable for our storage sku, let's conditionally choose a redundancy setting for our storage account by adding a new parameter globalRedundancy and combining it with the ternary operator. As part of that, we can get rid of our storageSku variable.

param location string = resourceGroup().location
param namePrefix string = 'stg'

param globalRedundancy bool = true // defaults to true, but can be overridden

var storageAccountName = '${namePrefix}${uniqueString(resourceGroup().id)}'

resource stg 'Microsoft.Storage/storageAccounts@2019-06-01' = {
    name: storageAccountName
    location: location
    kind: 'Storage'
    sku: {
        name: globalRedundancy ? 'Standard_GRS' : 'Standard_LRS' // if true --> GRS, else --> LRS
    }
}

output storageId string = stg.id

Next steps

In the next tutorial, we will learn how use the resource's symbolic name for simple property references and calculating dependencies:

4 - Using the symbolic resource name