Lambda functions for ARM templates

  • Article
  • 07/08/2024

This article describes the lambda functions to use in ARM templates. Lambda functions are essentially blocks of code that can be passed as an argument. They can take multiple parameters, but are restricted to a single line of code. In Bicep, lambda expression is in this format:

lambda(<lambda variable>, [<lambda variable>, ...], <expression>)

Tip

We recommend Bicep because it offers the same capabilities as ARM templates and the syntax is easier to use. To learn more, see deployment functions.

Limitations

ARM template lambda function has these limitations:

  • Lambda expression can only be specified directly as function arguments in these functions: filter(), groupBy(), map(), mapValues(), reduce(), sort(), and toObject().
  • Using lambda variables (the temporary variables used in the lambda functions) inside resource or module array access isn't currently supported.
  • Using lambda variables inside the listKeys function isn't currently supported.
  • Using lambda variables inside the reference function isn't currently supported.

filter

filter(inputArray, lambda function)

Filters an array with a custom filtering function.

In Bicep, use the filter function.

Parameters

Parameter Required Type Description
inputArray Yes array The array to filter.
lambda function Yes expression The lambda function applied to each input array element. If false, the item is filtered out of the output array.

Return value

An array.

Examples

The following examples show how to use the filter function.

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "variables": {
    "dogs": [
      {
        "name": "Evie",
        "age": 5,
        "interests": [
          "Ball",
          "Frisbee"
        ]
      },
      {
        "name": "Casper",
        "age": 3,
        "interests": [
          "Other dogs"
        ]
      },
      {
        "name": "Indy",
        "age": 2,
        "interests": [
          "Butter"
        ]
      },
      {
        "name": "Kira",
        "age": 8,
        "interests": [
          "Rubs"
        ]
      }
    ]
  },
  "resources": [],
  "outputs": {
    "oldDogs": {
      "type": "array",
      "value": "[filter(variables('dogs'), lambda('dog', greaterOrEquals(lambdaVariables('dog').age, 5)))]"
    },
    "dogNameIndex": {
      "type": "array",
      "value": "[filter(variables('dogs'), lambda('val', 'i', and(less(lambdaVariables('i'), 2), equals(substring(lambdaVariables('val').name, 0, 1), 'C'))))]"
    }
  }
}

The outputs from the preceding example:

Name Type Value
oldDogs Array [{"name":"Evie","age":5,"interests":["Ball","Frisbee"]},{"name":"Kira","age":8,"interests":["Rubs"]}]
dogNameIndex Array [{"name":"Casper","age":3,"interests":["Other dogs"]}]

oldDogs lists the dogs that are five or older; dogNameIndex identifies the dogs whose index number is less than two and whose name starts with the letter "C".

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "variables": {
    "copy": [
      {
        "name": "itemForLoop",
        "count": "[length(range(0, 10))]",
        "input": "[range(0, 10)[copyIndex('itemForLoop')]]"
      }
    ]
  },
  "resources": [],
  "outputs": {
    "filteredLoop": {
      "type": "array",
      "value": "[filter(variables('itemForLoop'), lambda('i', greater(lambdaVariables('i'), 5)))]"
    },
    "isEven": {
      "type": "array",
      "value": "[filter(range(0, 10), lambda('i', equals(0, mod(lambdaVariables('i'), 2))))]"
    }
  }
}

The output from the preceding example:

Name Type Value
filteredLoop Array [6, 7, 8, 9]
isEven Array [0, 2, 4, 6, 8]

filterdLoop shows the numbers in an array that are greater than 5; and isEven shows the even numbers in the array.

groupBy

groupBy(inputArray, lambda expression)

Creates an object with array values from an array, using a grouping condition.

In Bicep, use the groupBy function.

Parameters

Parameter Required Type Description
inputArray Yes array The array for grouping.
lambda expression Yes expression The lambda expression is applied to each input array element, and group the elements using the grouping condition.

Return value

An object.

Examples

The following example shows how to use the groupBy function.

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "variables": {
    "inputArray": [
      "foo",
      "bar",
      "baz"
    ]
  },
  "resources": [],
  "outputs": {
    "outObject": {
      "type": "object",
      "value": "[groupBy(variables('inputArray'), lambda('x', substring(lambdaVariables('x'), 0, 1)))]"
    }
  }
}

The output from the preceding example shows the dogs that are five or older:

Name Type Value
outObject Object {"f":["foo"],"b":["bar","baz"]}

outObject shows an object that groups the array elements by their first letters.

map

map(inputArray, lambda function)

Applies a custom mapping function to each element of an array.

In Bicep, use the map function.

Parameters

Parameter Required Type Description
inputArray Yes array The array to map.
lambda function Yes expression The lambda function applied to each input array element, in order to generate the output array.

Return value

An array.

Example

The following example shows how to use the map function.

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "variables": {
    "dogs": [
      {
        "name": "Evie",
        "age": 5,
        "interests": [
          "Ball",
          "Frisbee"
        ]
      },
      {
        "name": "Casper",
        "age": 3,
        "interests": [
          "Other dogs"
        ]
      },
      {
        "name": "Indy",
        "age": 2,
        "interests": [
          "Butter"
        ]
      },
      {
        "name": "Kira",
        "age": 8,
        "interests": [
          "Rubs"
        ]
      }
    ]
  },
  "resources": [],
  "outputs": {
    "dogNames": {
      "type": "array",
      "value": "[map(variables('dogs'), lambda('dog', lambdaVariables('dog').name))]"
    },
    "sayHi": {
      "type": "array",
      "value": "[map(variables('dogs'), lambda('dog', format('Hello {0}!', lambdaVariables('dog').name)))]"
    },
    "mapArray": {
      "type": "array",
      "value": "[map(range(0, length(variables('dogs'))), lambda('i', createObject('i', lambdaVariables('i'), 'dog', variables('dogs')[lambdaVariables('i')].name, 'greeting', format('Ahoy, {0}!', variables('dogs')[lambdaVariables('i')].name))))]"
    },
    "mapArrayIndex": {
      "type": "array",
      "value": "[map(variables('dogs'), lambda('x', 'i', createObject('index', lambdaVariables('i'), 'val', lambdaVariables('x').name)))]"
    }
  }
}

The output from the preceding example is:

Name Type Value
dogNames Array ["Evie","Casper","Indy","Kira"]
sayHi Array ["Hello Evie!","Hello Casper!","Hello Indy!","Hello Kira!"]
mapArray Array [{"i":0,"dog":"Evie","greeting":"Ahoy, Evie!"},{"i":1,"dog":"Casper","greeting":"Ahoy, Casper!"},{"i":2,"dog":"Indy","greeting":"Ahoy, Indy!"},{"i":3,"dog":"Kira","greeting":"Ahoy, Kira!"}]
mapArrayIndex Array [{"index":0,"val":"Evie"},{"index":1,"val":"Casper"},{"index":2,"val":"Indy"},{"index":3,"val":"Kira"}]

dogNames shows the dog names from the array of objects; sayHi concatenates "Hello" and each of the dog names; mapArray and mapArrayIndex create another two arrays of objects.

mapValues

mapValues(inputObject, lambda expression)

Creates an object from an input object, using a lambda expression to map values.

In Bicep, use the mapValues function.

Parameters

Parameter Required Type Description
inputObject Yes object The object to map.
lambda expression Yes expression The lambda expression used to map the values.

Return value

An object.

Example

The following example shows how to use the mapValues function.

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "variables": {
    "inputObject": {
      "foo": "foo",
      "bar": "bar"
    }
  },
  "resources": [],
  "outputs": {
    "mapObject": {
      "type": "object",
      "value": "[mapValues(variables('inputObject'), lambda('val', toUpper(lambdaVariables('val'))))]"
    }
  }
}

The output from the preceding example is:

Name Type Value
mapObject Object {foo: 'FOO', bar: 'BAR'}

mapObject creates another object with the values in upper case.

reduce

reduce(inputArray, initialValue, lambda function)

Reduces an array with a custom reduce function.

In Bicep, use the reduce function.

Parameters

Parameter Required Type Description
inputArray Yes array The array to reduce.
initialValue Yes any Initial value.
lambda function Yes expression The lambda function used to aggregate the current value and the next value.

Return value

Any.

Example

The following examples show how to use the reduce function.

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "variables": {
    "dogs": [
      {
        "name": "Evie",
        "age": 5,
        "interests": [
          "Ball",
          "Frisbee"
        ]
      },
      {
        "name": "Casper",
        "age": 3,
        "interests": [
          "Other dogs"
        ]
      },
      {
        "name": "Indy",
        "age": 2,
        "interests": [
          "Butter"
        ]
      },
      {
        "name": "Kira",
        "age": 8,
        "interests": [
          "Rubs"
        ]
      }
    ],
    "ages": "[map(variables('dogs'), lambda('dog', lambdaVariables('dog').age))]"
  },
  "resources": [],
  "outputs": {
    "totalAge": {
      "type": "int",
      "value": "[reduce(variables('ages'), 0, lambda('cur', 'next', add(lambdaVariables('cur'), lambdaVariables('next'))))]"
    },
    "totalAgeAdd1": {
      "type": "int",
      "value": "[reduce(variables('ages'), 1, lambda('cur', 'next', add(lambdaVariables('cur'), lambdaVariables('next'))))]"
    },
    "oddAge": {
      "type": "int",
      "value": "[reduce(variables('ages'), 0, lambda('cur', 'next', 'i', if(equals(mod(lambdaVariables('i'), 2), 0), add(lambdaVariables('cur'), lambdaVariables('next')), lambdaVariables('cur'))))]"
    }
  }
}

The output from the preceding example is:

Name Type Value
totalAge int 18
totalAgeAdd1 int 19
oddAge int 7

totalAge sums the ages of the dogs; totalAgeAdd1 has an initial value of 1, and adds all the dog ages to the initial values. oddAge sums the ages of dogs that are located at even indices, specifically 5 (Evie) and 2 (Indy).

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "resources": [],
  "outputs": {
    "reduceObjectUnion": {
      "type": "object",
      "value": "[reduce(createArray(createObject('foo', 123), createObject('bar', 456), createObject('baz', 789)), createObject(), lambda('cur', 'next', union(lambdaVariables('cur'), lambdaVariables('next'))))]"
    }
  }
}

The output from the preceding example is:

Name Type Value
reduceObjectUnion object {"foo":123,"bar":456,"baz":789}

The union function returns a single object with all elements from the parameters. The function call unionizes the key value pairs of the objects into a new object.

sort

sort(inputArray, lambda function)

Sorts an array with a custom sort function.

In Bicep, use the sort function.

Parameters

Parameter Required Type Description
inputArray Yes array The array to sort.
lambda function Yes expression The lambda function used to compare two array elements for ordering. If true, the second element will be ordered after the first in the output array.

Return value

An array.

Example

The following example shows how to use the sort function.

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "variables": {
    "dogs": [
      {
        "name": "Evie",
        "age": 5,
        "interests": [
          "Ball",
          "Frisbee"
        ]
      },
      {
        "name": "Casper",
        "age": 3,
        "interests": [
          "Other dogs"
        ]
      },
      {
        "name": "Indy",
        "age": 2,
        "interests": [
          "Butter"
        ]
      },
      {
        "name": "Kira",
        "age": 8,
        "interests": [
          "Rubs"
        ]
      }
    ]
  },
  "resources": [],
  "outputs": {
    "dogsByAge": {
      "type": "array",
      "value": "[sort(variables('dogs'), lambda('a', 'b', less(lambdaVariables('a').age, lambdaVariables('b').age)))]"
    }
  }
}

The output from the preceding example sorts the dog objects from the youngest to the oldest:

Name Type Value
dogsByAge Array [{"name":"Indy","age":2,"interests":["Butter"]},{"name":"Casper","age":3,"interests":["Other dogs"]},{"name":"Evie","age":5,"interests":["Ball","Frisbee"]},{"name":"Kira","age":8,"interests":["Rubs"]}]

toObject

toObject(inputArray, lambda function, [lambda function])

Converts an array to an object with a custom key function and optional custom value function. See items about converting an object to an array.

In Bicep, use the toObject function.

Parameters

Parameter Required Type Description
inputArray Yes array The array used for creating an object.
lambda function Yes expression The lambda function used to provide the key predicate.
lambda function No expression The lambda function used to provide the value predicate.

Return value

An object.

Example

The following example shows how to use the toObject function with the two required parameters:

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "variables": {
    "dogs": [
      {
        "name": "Evie",
        "age": 5,
        "interests": [
          "Ball",
          "Frisbee"
        ]
      },
      {
        "name": "Casper",
        "age": 3,
        "interests": [
          "Other dogs"
        ]
      },
      {
        "name": "Indy",
        "age": 2,
        "interests": [
          "Butter"
        ]
      },
      {
        "name": "Kira",
        "age": 8,
        "interests": [
          "Rubs"
        ]
      }
    ]
  },
  "resources": [],
  "outputs": {
    "dogsObject": {
      "type": "object",
      "value": "[toObject(variables('dogs'), lambda('entry', lambdaVariables('entry').name))]"
    }
  }
}

The preceding example generates an object based on an array.

Name Type Value
dogsObject Object {"Evie":{"name":"Evie","age":5,"interests":["Ball","Frisbee"]},"Casper":{"name":"Casper","age":3,"interests":["Other dogs"]},"Indy":{"name":"Indy","age":2,"interests":["Butter"]},"Kira":{"name":"Kira","age":8,"interests":["Rubs"]}}

The following toObject function with the third parameter provides the same output.

"outputs": {
  "dogsObject": {
    "type": "object",
    "value": "[toObject(variables('dogs'), lambda('entry', lambdaVariables('entry').name), lambda('entry', lambdaVariables('entry')))]"
  }
}

The following example shows how to use the toObject function with three parameters.

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "variables": {
    "dogs": [
      {
        "name": "Evie",
        "properties": {
          "age": 5,
          "interests": [
            "Ball",
            "Frisbee"
          ]
        }
      },
      {
        "name": "Casper",
        "properties": {
          "age": 3,
          "interests": [
            "Other dogs"
          ]
        }
      },
      {
        "name": "Indy",
        "properties": {
          "age": 2,
          "interests": [
            "Butter"
          ]
        }
      },
      {
        "name": "Kira",
        "properties": {
          "age": 8,
          "interests": [
            "Rubs"
          ]
        }
      }
    ]
  },
  "resources": [],
  "outputs": {
    "dogsObject": {
      "type": "object",
      "value": "[toObject(variables('dogs'), lambda('entry', lambdaVariables('entry').name), lambda('entry', lambdaVariables('entry').properties))]"
    }
  }
}

The preceding example generates an object based on an array.

Name Type Value
dogsObject Object {"Evie":{"age":5,"interests":["Ball","Frisbee"]},"Casper":{"age":3,"interests":["Other dogs"]},"Indy":{"age":2,"interests":["Butter"]},"Kira":{"age":8,"interests":["Rubs"]}}

Next steps