github.com/pgray/terraform@v0.5.4-0.20170822184730-b6a464c5214d/website/docs/configuration/locals.html.md (about)

     1  ---
     2  layout: "docs"
     3  page_title: "Configuring Local Values"
     4  sidebar_current: "docs-config-locals"
     5  description: |-
     6    Local values assign a name to an expression that can then be used multiple times
     7    within a module.
     8  ---
     9  
    10  # Local Value Configuration
    11  
    12  Local values assign a name to an expression, that can then be used multiple
    13  times within a module.
    14  
    15  Comparing modules to functions in a traditional programming language,
    16  if [variables](./variables.html) are analogous to function arguments and
    17  [outputs](./outputs.html) are analogous to function return values then
    18  _local values_ are comparable to a function's local variables.
    19  
    20  This page assumes you're already familiar with
    21  [the configuration syntax](/docs/configuration/syntax.html).
    22  
    23  ## Example
    24  
    25  Local values are defined in `locals` blocks:
    26  
    27  ```hcl
    28  # Ids for multiple sets of EC2 instances, merged together
    29  locals {
    30    instance_ids = "${concat(aws_instance.blue.*.id, aws_instance.green.*.id)}"
    31  }
    32  
    33  # A computed default name prefix
    34  locals {
    35    default_name_prefix = "${var.project_name}-web"
    36    name_prefix         = "${var.name_prefix != "" ? var.name_prefix : local.default_name_prefix}"
    37  }
    38  ```
    39  
    40  ## Description
    41  
    42  The `locals` block defines one or more local variables within a module.
    43  Each `locals` block can have as many locals as needed, and there can be any
    44  number of `locals` blocks within a module.
    45  
    46  The names given for the items in the `locals` block must be unique throughout
    47  a module. The given value can be any expression that is valid within
    48  the current module.
    49  
    50  The expression of a local value can refer to other locals, but as usual
    51  reference cycles are not allowed. That is, a local cannot refer to itself
    52  or to a variable that refers (directly or indirectly) back to it.
    53  
    54  It's recommended to group together logically-related local values into
    55  a single block, particulary if they depend on each other. This will help
    56  the reader understand the relationships between variables. Conversely,
    57  prefer to define _unrelated_ local values in _separate_ blocks, and consider
    58  annotating each block with a comment describing any context common to all
    59  of the enclosed locals.