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Errand’s expression syntax is defined by HCL. Expressions appear in computed block expression attributes, task condition and working_dir values, and inside ${} interpolations in command strings.

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Operators

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Arithmetic

OperatorOperation
+Addition
-Subtraction
*Multiplication
/Division
%Remainder
computed "workers" {
  expression = var.cpu_count * 2
}

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Comparison

OperatorOperation
==Equal
!=Not equal
<Less than
<=Less than or equal
>Greater than
>=Greater than or equal
task "deploy" {
  condition = var.replicas > 0
  commands  = ["kubectl apply -f k8s/"]
}

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Logical

OperatorOperation
&&And
||Or
!Not
task "migrate" {
  condition = var.env == "production" && var.run_migrations
  commands  = ["./bin/migrate up"]
}

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Precedence

Operators evaluate in this order, highest to lowest:
PriorityOperators
Highest!, - (unary)
*, /, %
+, -
>, >=, <, <=
==, !=
&&
Lowest||
Use parentheses when order might be ambiguous: 
condition = (var.env == "production" || var.env == "staging") && var.ready

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Conditional

The ternary operator selects between two values based on a condition: 
condition ? value_if_true : value_if_false

task "build" {
  commands = [
    "go build ${var.debug ? "-race" : ""} ./...",
  ]
}
Both branches must produce a compatible type.

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Index and attribute access

Access list elements by zero-based index: 
expression = var.services[0]
Access map and object values by key or dot notation: 
expression = var.ports["api"]
expression = var.db.host
Dot notation and bracket notation are equivalent for string keys. Accessing an out-of-bounds index or a missing key is an error at evaluation time.

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Function calls

Call a function by name with arguments in parentheses. Functions can be nested: 
expression = lower(replace(var.branch, "/", "-"))
All built-in functions are available in every expression context. See the functions reference.

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HCL expression language

Errand uses a subset of HCL’s expression language. For expressions, splat expressions ([*]), and template directives (%{if}, %{for}) are not supported (yet).