github.com/muratcelep/terraform@v1.1.0-beta2-not-internal-4/website/docs/language/functions/can.html.md (about)

     1  ---
     2  layout: "language"
     3  page_title: "can - Functions - Configuration Language"
     4  sidebar_current: "docs-funcs-conversion-can"
     5  description: |-
     6    The can function tries to evaluate an expression given as an argument and
     7    indicates whether the evaluation succeeded.
     8  ---
     9  
    10  # `can` Function
    11  
    12  `can` evaluates the given expression and returns a boolean value indicating
    13  whether the expression produced a result without any errors.
    14  
    15  This is a special function that is able to catch errors produced when evaluating
    16  its argument. For most situations where you could use `can` it's better to use
    17  [`try`](./try.html) instead, because it allows for more concise definition of
    18  fallback values for failing expressions.
    19  
    20  The primary purpose of `can` is to turn an error condition into a boolean
    21  validation result when writing
    22  [custom variable validation rules](/docs/language/values/variables.html#custom-validation-rules).
    23  For example:
    24  
    25  ```
    26  variable "timestamp" {
    27    type        = string
    28  
    29    validation {
    30      # formatdate fails if the second argument is not a valid timestamp
    31      condition     = can(formatdate("", var.timestamp))
    32      error_message = "The timestamp argument requires a valid RFC 3339 timestamp."
    33    }
    34  }
    35  ```
    36  
    37  The `can` function can only catch and handle _dynamic_ errors resulting from
    38  access to data that isn't known until runtime. It will not catch errors
    39  relating to expressions that can be proven to be invalid for any input, such
    40  as a malformed resource reference.
    41  
    42  ~> **Warning:** The `can` function is intended only for simple tests in
    43  variable validation rules. Although it can technically accept any sort of
    44  expression and be used elsewhere in the configuration, we recommend against
    45  using it in other contexts. For error handling elsewhere in the configuration,
    46  prefer to use [`try`](./try.html).
    47  
    48  ## Examples
    49  
    50  ```
    51  > local.foo
    52  {
    53    "bar" = "baz"
    54  }
    55  > can(local.foo.bar)
    56  true
    57  > can(local.foo.boop)
    58  false
    59  ```
    60  
    61  The `can` function will _not_ catch errors relating to constructs that are
    62  provably invalid even before dynamic expression evaluation, such as a malformed
    63  reference or a reference to a top-level object that has not been declared:
    64  
    65  ```
    66  > can(local.nonexist)
    67  
    68  Error: Reference to undeclared local value
    69  
    70  A local value with the name "nonexist" has not been declared.
    71  ```
    72  
    73  ## Related Functions
    74  
    75  * [`try`](./try.html), which tries evaluating a sequence of expressions and
    76    returns the result of the first one that succeeds.