packer/website/content/docs/templates/hcl_templates/functions/index.mdx

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page_title: Built-in functions reference overview
description: |-
  Functions built into HCL language can transform and combine values. Learn about using built-in functions in your Packer templates. 
---

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# Built-in functions overview

This topic provides an overview of the built-in HashiCorp configuration language (HCL) functions you can use in your Packer templates. 

## Functions syntax

You can use functions in Packer templates to transform and combine values. The 
syntax for function calls is a function name followed by comma-separated
arguments in parentheses:

```hcl
max(5, 12, 9)
```

For information on invoking functions in string literals, refer to the
[string literals reference](/packer/docs/templates/hcl_templates/expressions#string-literals) section.

HCL does not support user-defined functions. You can only call
the functions built into the language.