Parsing, encoding and decoding of HCL to and from Go types

This package provides idiomatic Go functions for marshalling and unmarshalling HCL, as well as an AST

It supports the same tags as the Hashicorp hcl2 gohcl package, but is much less complex.

Unlike gohcl it also natively supports time.Duration, time.Time, encoding.TextUnmarshaler and json.Unmarshaler.

It is HCL1 compatible and does not support any HCL2 specific features.

Design

HCL -> AST -> Go -> AST -> HCL

Mapping can start from any point in this cycle.

Marshalling, unmarshalling, parsing and serialisation are all structurally isomorphic operations. That is, HCL can be deserialised into an AST or Go, or vice versa, and the structure on both ends will be identical.

HCL is always parsed into an AST before unmarshaling and, similarly, Go structures are always mapped to an AST before being serialised to HCL.

Schema reflection

HCL has no real concept of schemas (that I can find), but there is precedent for something similar in Terraform variable definition files. This package supports reflecting a rudimentary schema from Go, where the value for each attribute is one of the scalar types number, string or boolean. Lists and maps are typed by example.

Here's an example schema.

// A string field.
str = string
num = number
bool = boolean
list = [string]

// A map.
map = {
  string: number,
}

// A block.
block "name" {
  attr = string
}

// Repeated blocks.
block_slice "label0" "label1" {
  attr = string
}

Comments are from help:"" tags. See schema_test.go for details.

Struct field tags

The tag format is as with other similar serialisation packages:

hcl:"[<name>][,<option>]"

The supported options are:

Additionally, a separate help:"" tag can be specified to populate comment fields in the AST when serialising Go structures.

Position

Any block with a field named Pos of the type hcl.Position will have that field populated with positional information:

Pos Position `hcl:"-"`