hcl/ext/typeexpr

HCL Type Expressions Extension

This HCL extension defines a convention for describing HCL types using function call and variable reference syntax, allowing configuration formats to include type information provided by users.

The type syntax is processed statically from a hcl.Expression, so it cannot use any of the usual language operators. This is similar to type expressions in statically-typed programming languages.

variable "example" {
  type = list(string)
}

The extension is built using the hcl.ExprAsKeyword and hcl.ExprCall functions, and so it relies on the underlying syntax to define how "keyword" and "call" are interpreted. The above shows how they are interpreted in the HCL native syntax, while the following shows the same information expressed in JSON:

{
  "variable": {
    "example": {
      "type": "list(string)"
    }
  }
}

Notice that since we have additional contextual information that we intend to allow only calls and keywords the JSON syntax is able to parse the given string directly as an expression, rather than as a template as would be the case for normal expression evaluation.

For more information, see the godoc reference.

Type Expression Syntax

When expressed in the native syntax, the following expressions are permitted in a type expression:

• string - string
• bool - boolean
• number - number
• any - cty.DynamicPseudoType (in function TypeConstraint only)
• list(<type_expr>) - list of the type given as an argument
• set(<type_expr>) - set of the type given as an argument
• map(<type_expr>) - map of the type given as an argument
• tuple([<type_exprs...>]) - tuple with the element types given in the single list argument
• object({<attr_name>=<type_expr>, ...} - object with the attributes and corresponding types given in the single map argument

For example:

• list(string)
• object({name=string,age=number})
• map(object({name=string,age=number}))

Note that the object constructor syntax is not fully-general for all possible object types because it requires the attribute names to be valid identifiers. In practice it is expected that any time an object type is being fixed for type checking it will be one that has identifiers as its attributes; object types with weird attributes generally show up only from arbitrary object constructors in configuration files, which are usually treated either as maps or as the dynamic pseudo-type.

Type Constraints as Values

Along with defining a convention for writing down types using HCL expression constructs, this package also includes a mechanism for representing types as values that can be used as data within an HCL-based language.

typeexpr.TypeConstraintType is a cty capsule type that encapsulates cty.Type values. You can construct such a value directly using the TypeConstraintVal function:

tyVal := typeexpr.TypeConstraintVal(cty.String)

// We can unpack the type from a value using TypeConstraintFromVal
ty := typeExpr.TypeConstraintFromVal(tyVal)

However, the primary purpose of typeexpr.TypeConstraintType is to be specified as the type constraint for an argument, in which case it serves as a signal for HCL to treat the argument expression as a type constraint expression as defined above, rather than as a normal value expression.

"An argument" in the above in practice means the following two locations:

• As the type constraint for a parameter of a cty function that will be used in an hcl.EvalContext. In that case, function calls in the HCL native expression syntax will require the argument to be valid type constraint expression syntax and the function implementation will receive a TypeConstraintType value as the argument value for that parameter.

• As the type constraint for a hcldec.AttrSpec or hcldec.BlockAttrsSpec when decoding an HCL body using hcldec. In that case, the attributes with that type constraint will be required to be valid type constraint expression syntax and the result will be a TypeConstraintType value.

Note that the special handling of these arguments means that an argument marked in this way must use the type constraint syntax directly. It is not valid to pass in a value of TypeConstraintType that has been obtained dynamically via some other expression result.

TypeConstraintType is provided with the intent of using it internally within application code when incorporating type constraint expression syntax into an HCL-based language, not to be used for dynamic "programming with types". A calling application could support programming with types by defining its own capsule type, but that is not the purpose of TypeConstraintType.

The "convert" cty Function

Building on the TypeConstraintType described in the previous section, this package also provides typeexpr.ConvertFunc which is a cty function that can be placed into a cty.EvalContext (conventionally named "convert") in order to provide a general type conversion function in an HCL-based language:

  foo = convert("true", bool)

The second parameter uses the mechanism described in the previous section to require its argument to be a type constraint expression rather than a value expression. In doing so, it allows converting with any type constraint that can be expressed in this package's type constraint syntax. In the above example, the foo argument would receive a boolean true, or cty.True in cty terms.

The target type constraint must always be provided statically using inline type constraint syntax. There is no way to dynamically select a type constraint using this function.