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Implement Your Own Tool Using HCL (Part 2)

by WeakPixelApril 13th, 2022

Too Long; Didn't Read

This is the second part of my HCL series. You may find the first part here (Part 1) In the second post of my HCL series I want to extend our example with: Cobra Commandline Variables Functions Cobra Cobra is my favorite library to build command-line tools. We start off with the example program from the first post (source).

featured image - Implement Your Own Tool Using HCL (Part 2)

This is the second part of my HCL series.

You may find the first part here (Part 1)

In the second post of my HCL series I want to extend our example with:

Cobra Commandline
Variables
Functions

Cobra

Cobra is my favorite library to build command-line tools.

We start off with the example program from the first post (source).

As I write before I want to introduce you to the Cobra command-line tool. In order to use it we have to add a new import:

import (
    "fmt"
    "os"

    "github.com/spf13/cobra"
    // ...

Next rename the main() function to newRunCommand() and refactor it to return a cobra.Command

func newRunCommand() *cobra.Command {
    // contains all variables given by the user with --var "key=value"
    vars := []string{}
    cmd := cobra.Command{
        Use: "run"
        Short: "Executes tasks",
        RunE: func(cmd *cobra.Command, args []string) error {
            config := &Config{}
            err := hclsimple.Decode("example.hcl", []byte(exampleHCL), nil, config)
            if err != nil {
                return err
            }
            for _, task := range config.Tasks {
                fmt.Printf("Task: %s\n", task.Name)
                for _, step := range task.Steps {
                    fmt.Printf("    Step: %s %s\n", step.Type, step.Name)
                    var runner Runner
                    switch step.Type {
                    case "mkdir":
                        runner = &MkdirStep{}
                    case "exec":
                        runner = &ExecStep{}
                    default:
                        return fmt.Errorf("unknown step type %q", step.Type)
                    }

                    diags := gohcl.DecodeBody(step.Remain, nil, runner)
                    if diags.HasErrors() {
                        return diags
                    }
                    err = runner.Run()
                    if err != nil {
                        return err
                    }
                }
            }

            return nil
        },
    }
    // Define an optional "var" flag for the commnd
    cmd.Flags().StringArrayVar(&vars, "var", nil, "Sets variable. Format <name>=<value>")
    return &cmd
}

The Use field describes the subcommand name. The Short field allows defining a short command description.

The RunE implements the execution of the (sub-)command. It contains our HCL parsing code. Since RunE allows us to return an error we also have refactored the code to just return an error instead of using os.Exit(1).

After that we implement a new main function looking like:

func main() {
    root := cobra.Command{
        Use: "taskexec",
    }
    root.AddCommand(newRunCommand())
    err := root.Execute()
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }
}

The root command is just an empty cobra.Command. To the root command we add our subcommand with root.AddCommand(newRunCommand()).

Let's try out what happens if we run our program:

go run main.go 
Usage:
  taskexec [command]

Available Commands:
  completion  Generate the autocompletion script for the specified shell
  help        Help about any command
  run         Executes tasks

Flags:
  -h, --help   help for taskexec

Let's try to show the help for the subcommand:

go run main.go run -h
Executes tasks

Usage:
  taskexec run [flags]

Flags:
  -h, --help              help for run
      --var stringArray   Sets variable. Format <name>=<value>

Great! Next, we want to make use of the variables. To use variables in our HCL config, we must learn about the hcl.EvalContext

EvalContext

The hcl.EvalContext allows as to define variables and functions.

type EvalContext struct {
    Variables map[string]cty.Value
    Functions map[string]function.Function
}

For now, we focus on the variables. The Variables map allows us to define the variable name as key and as value a cty.Value. The cty.Value is part of the github.com/zclconf/go-cty/cty package. The package provides a dynamic type system.

You can read more about cty on the github project.

Let's come back to hcl.EvalContext. Where is this context struct actually used? In our example code we have two instances:

hclsimple.Decode("example.hcl", []byte(exampleHCL), 
    /*&hcl.EvalContext{}*/ nil, config)

and

diags := gohcl.DecodeBody(step.Remain,
     /*&hcl.EvalContext{}*/ nil, runner)

Variables

In our command we have defined a vars slice which contains the user-defined variables in the format:

--var "key=value" ...

So let's get started and create hcl.EvalContext and populate it with the vars parameters from the command line.

func newEvalContext(vars []string) (*hcl.EvalContext, error) {
    varMap := map[string]cty.Value{}
    for _, v := range vars {
        el := strings.Split(v, "=")
        if len(el) != 2 {
            return nil, fmt.Errorf("invalid format: %s", v)
        }
        varMap[el[0]] = cty.StringVal(el[1])
    }

    ctx := &hcl.EvalContext{}
    ctx.Variables = map[string]cty.Value{
        "var": cty.ObjectVal(varMap),
    }
    return ctx, nil
}

We use the newEvalContext() function in our subcommand to create the EvalContext and use the context in all places where we decode the HCL document:

// ...
RunE: func(cmd *cobra.Command, args []string) error {
    ctx, err := newEvalContext(vars)
    if err != nil {
        return err
    }
    config := &Config{}
    err = hclsimple.Decode("example.hcl", []byte(exampleHCL), ctx, config)
    // ...
    for _, task := range config.Tasks {
        fmt.Printf("Task: %s\n", task.Name)
        for _, step := range task.Steps {
            // ...
            diags := gohcl.DecodeBody(step.Remain, ctx, runner)
            // ...
        }
    }

    return nil
},
// ...

And finally, we change our exampleHCL to make use of variables:

exampleHCL = `
    task "first_task" {
        step "mkdir" "build_dir" {
            path = var.buildDir
        }
        step "exec" "list_build_dir" {
            command = "ls ${var.buildDir}"
        }
    }
`

Let's try to execute the command without defining the buildDir variable:

go run main.go run 
...
example.hcl:4,15-24: Unsupported attribute; This object does not have an attribute named "buildDir"., and 1 other diagnostic(s)
exit status 1

Good, it fails with a detailed error message.

Now we try to execute the command with the needed variable:

go run main.go run --var buildDir=./build
Task: first_task
    Step: mkdir build_dir
    Step: exec list_build_dir

And it works as expected!

You can see the full source code here

Functions

Next, we want to explore how e.g. Terraform provides these nice inline functions which makes life so much easier to deal with input variables. It might not make much sense in our example but let's try to implement a function that converts all cased letters into uppercase:

helloValue = "${upper("hello")} World"

To implement a function we must add a new module to our import "github.com/zclconf/go-cty/cty/function".

We have to use the function.Spec struct to create with function.New our function implementation:

var upperFn = function.New(&function.Spec{
    // Define the required parameters.
    Params: []function.Parameter{
        {
            Name:             "str",
            Type:             cty.String,
            AllowDynamicType: true,
        },
    },
    // Define the return type
    Type: function.StaticReturnType(cty.String),
    // Function implementation:
    Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
        in := args[0].AsString()
        out := strings.ToUpper(in)
        return cty.StringVal(out), nil
    },
})

And last we add the new function to our EvalContext:

func newEvalContext(vars []string) (*hcl.EvalContext, error) {
    
    // ...

    ctx.Functions = map[string]function.Function{
        "upper": upperFn,
    }
    return ctx, nil
}

Update the exampleHCL to make use of our brand new define function:

exampleHCL = `
    task "first_task" {
        step "mkdir" "build_dir" {
            path = upper(var.buildDir)
        }
        step "exec" "list_build_dir" {
            command = "ls ${ upper(var.buildDir) }"
        }
    }
`

Add some debug output to our example Step execution (mkdir, exec) and run the program:

go run main.go run --var "buildDir=./build"

Task: first_task
    Step: mkdir build_dir
        Path:./build
    Step: exec list_build_dir
        Command: ls ./BUILD

and as expected we have an upper case build directory.

If you don't want to implement all the functions yourself or you need some inspiration to implement a function you find want you looking for here:

Resources

Resources:

Full Source Code

package main

import (
    "fmt"
    "os"
    "strings"

    "github.com/spf13/cobra"
    "github.com/zclconf/go-cty/cty"

    "github.com/hashicorp/hcl/v2"
    "github.com/hashicorp/hcl/v2/gohcl"
    "github.com/hashicorp/hcl/v2/hclsimple"

    "github.com/zclconf/go-cty/cty/function"
)

var (
    exampleHCL = `
        task "first_task" {
            step "mkdir" "build_dir" {
                path = upper(var.buildDir)
            }
            step "exec" "list_build_dir" {
                command = "ls ${ upper(var.buildDir) }"
            }
        }
    `
)

func main() {
    root := cobra.Command{
        Use: "taskexec",
    }
    root.AddCommand(newRunCommand())
    err := root.Execute()
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }
}

func newRunCommand() *cobra.Command {
    vars := []string{}
    cmd := cobra.Command{
        Use:   "run",
        Short: "Executes tasks",
        RunE: func(cmd *cobra.Command, args []string) error {
            ctx, err := newEvalContext(vars)
            if err != nil {
                return err
            }
            config := &Config{}
            err = hclsimple.Decode("example.hcl", []byte(exampleHCL), ctx, config)
            if err != nil {
                return err
            }
            for _, task := range config.Tasks {
                fmt.Printf("Task: %s\n", task.Name)
                for _, step := range task.Steps {
                    fmt.Printf("    Step: %s %s\n", step.Type, step.Name)
                    var runner Runner
                    switch step.Type {
                    case "mkdir":
                        runner = &MkdirStep{}
                    case "exec":
                        runner = &ExecStep{}
                    default:
                        return fmt.Errorf("unknown step type %q", step.Type)
                    }

                    diags := gohcl.DecodeBody(step.Remain, ctx, runner)
                    if diags.HasErrors() {
                        return diags
                    }
                    err = runner.Run()
                    if err != nil {
                        return err
                    }
                }
            }

            return nil
        },
    }

    cmd.Flags().StringArrayVar(&vars, "var", nil, "Sets variable. Format <name>=<value>")

    return &cmd
}

func newEvalContext(vars []string) (*hcl.EvalContext, error) {
    varMap := map[string]cty.Value{}
    for _, v := range vars {
        el := strings.Split(v, "=")
        if len(el) != 2 {
            return nil, fmt.Errorf("invalid format: %s", v)
        }
        varMap[el[0]] = cty.StringVal(el[1])
    }

    ctx := &hcl.EvalContext{}
    ctx.Variables = map[string]cty.Value{
        "var": cty.ObjectVal(varMap),
    }
    ctx.Functions = map[string]function.Function{
        "upper": upperFn,
    }
    return ctx, nil
}

var upperFn = function.New(&function.Spec{
    // Define the required parameters.
    Params: []function.Parameter{
        {
            Name:             "str",
            Type:             cty.String,
            AllowDynamicType: true,
        },
    },
    // Define the return type
    Type: function.StaticReturnType(cty.String),
    // Function implementation:
    Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
        in := args[0].AsString()
        out := strings.ToUpper(in)
        return cty.StringVal(out), nil
    },
})

type Config struct {
    Tasks []*Task `hcl:"task,block"`
}
type Task struct {
    Name  string  `hcl:"name,label"`
    Steps []*Step `hcl:"step,block"`
}

type Step struct {
    Type   string   `hcl:"type,label"`
    Name   string   `hcl:"name,label"`
    Remain hcl.Body `hcl:",remain"`
}

type ExecStep struct {
    Command string `hcl:"command"`
}

func (s *ExecStep) Run() error {
    fmt.Println("\tCommand: " + s.Command)
    return nil
}

type MkdirStep struct {
    Path string `hcl:"path"`
}

func (s *MkdirStep) Run() error {
    fmt.Println("\tPath:" + s.Path)
    return nil
}

type Runner interface {
    Run() error
}