Go bindings for Nickel

With these golang bindings, you can use the Nickel configuration language directly from go as part of your single static executable.

Getting started

Add Nickel bindings to your project using go get:

go get github.com/nickel-lang/go-nickel@main

(You need to use main because we haven't had any releases yet. Also, bindings are only available for linux amd64 so far. This will change soon.)

Then you can evaluate Nickel strings from within your go code:

import (
  "fmt"
  "github.com/nickel-lang/go-nickel"
)

func main() {
  ctx := nickel.NewContext()
	expr, _ := ctx.EvalDeep(`
  {
    port = 80,
    name = "myserver"
  } | { port | Number, name | String }
  `)
  record, _ := expr.ToRecord()
  name, _ := record["name"].ToString()

  fmt.Printf("The server name is `%s`", name)
}

Unmarshalling support

These bindings have support for go's native unmarshalling:

import (
  "fmt"
  "github.com/nickel-lang/go-nickel"
)

type Config struct {
	Port int    `json:"port"`
	Name string `json:"name"`
}

func main() {
  ctx := nickel.NewContext()
	expr, _ := ctx.EvalDeep(`
  {
    port = 80,
    name = "myserver"
  } | { port | Number, name | String }
  `)
  var config Config
  _ = expr.ConvertTo(&config)
  fmt.Printf("The config is `%v`", config)
}

Lazy (shallow) evaluation

Lazy evaluation is a key feature of Nickel, as it allows you to evaluate only a small part of a large configuration. These go bindings support shallow evaluation. Evaluating containers (record and arrays) shallowly means that you delay evaluating the elements.

import (
  "fmt"
  "github.com/nickel-lang/go-nickel"
)

func main() {
  ctx := nickel.NewContext()
	expr, _ := ctx.EvalShallow(`{
    port = 79 + 1,
    name = "my" ++ "server"
  }`)

  // The top-level expression is a record.
  record, _ := expr.ToRecord()

  // The record has "port" and "name" fields.
  port := record["port"]

  // The port isn't a number yet, it's the (unevaluated) expression 79 + 1
  if port.IsNumber() {
    fmt.Print("it shouldn't be a number yet!")
    return
  }

  portVal, _ := port.EvalShallow()
  portNum, _ := portVal.ToInt64()
  fmt.Printf("now it's a number: %d\n", portNum)
}