github.com/Ilhicas/nomad@v1.0.4-0.20210304152020-e86851182bc3/website/content/docs/job-specification/hcl2/functions/conversion/can.mdx (about)

     1  ---
     2  layout: docs
     3  page_title: can - Functions - Configuration Language
     4  sidebar_title: 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`](/docs/job-specification/hcl2/functions/conversion/try) instead, because it allows for more concise definition of
    18  fallback values for failing expressions.
    19  
    20  The `can` function can only catch and handle _dynamic_ errors resulting from
    21  access to data that isn't known until runtime. It will not catch errors
    22  relating to expressions that can be proven to be invalid for any input, such
    23  as a malformed reference.
    24  
    25  ~> **Warning:** The `can` function is intended only for simple tests in
    26  variable validation rules. Although it can technically accept any sort of
    27  expression and be used elsewhere in the configuration, we recommend against
    28  using it in other contexts. For error handling elsewhere in the configuration,
    29  prefer to use [`try`](/docs/job-specification/hcl2/functions/conversion/try).
    30  
    31  ## Examples
    32  
    33  ```shell-session
    34  > local.foo
    35  {
    36    "bar" = "baz"
    37  }
    38  > can(local.foo.bar)
    39  true
    40  > can(local.foo.boop)
    41  false
    42  ```
    43  
    44  The `can` function will _not_ catch errors relating to constructs that are
    45  provably invalid even before dynamic expression evaluation, such as a malformed
    46  reference or a reference to a top-level object that has not been declared:
    47  
    48  ```shell-session
    49  > can(local.nonexist)
    50  
    51  Error: Reference to undeclared local value
    52  
    53  A local value with the name "nonexist" has not been declared.
    54  ```
    55  
    56  ## Related Functions
    57  
    58  - [`try`](/docs/job-specification/hcl2/functions/conversion/try), which tries evaluating a sequence of expressions and
    59    returns the result of the first one that succeeds.