Ego

Ego is an ERb style templating language for Go. It works by transpiling templates into pure Go and including them at compile time. These templates are light wrappers around the Go language itself.

Usage

To install ego:

$ go get github.com/benbjohnson/ego/...

Then run

ego

.ego

$ ego mypkg

How to Write Templates

An ego template lets you write text that you want to print out but gives you some handy tags to let you inject actual Go code. This means you don't need to learn a new scripting language to write ego templates—you already know Go!

Raw Text

Any text the

ego

 and 
%>
 tags is considered raw text.
If you have a template like this:
hello!
goodbye!

Then 
ego
 will generate a matching 
.ego.go
 file:
io.WriteString(w, "hello!\ngoodbye!")

Unfortunately that file won't run because we're missing a 
package
 line at the top.
We can fix that with code blocks.

Code Blocks

A code block is a section of your template wrapped in 
 and 
%>
 tags.
It is raw Go code that will be inserted into our generate 
.ego.go
 file as-is.

For example, given this template:
hello!
goodbye!


The 
ego
 tool will generate:
package myapp

import (
    "context"
    "io"
)
func Render(ctx context.Context, w io.Writer) {
    io.WriteString(w, "hello!\ngoodbye!")
}

Note the 
context
 and 
io
 packages are automatically imported to your template.
These are the only packages that do this.
You'll need to import any other packages you use.


Print Blocks

Our template is getting more useful.
We now have actually runnable Go code.
However, our templates typically need output text frequently so there are blocks specifically for this task called print blocks.
These print blocks wrap a Go expression with 
 and 
%>
 tags.

We can expand our previous example and add a type and fields to our code:
    hello, !

    goodbye, !

We now have a conditional around our 
Greet
 field and we are printing the 
Name
 field.
Our generated code will look like:
package myapp

import (
    "context"
    "io"
)
type NameRenderer struct {
    Name  string
    Greet bool
}
func Render(ctx context.Context, w io.Writer) {
    if r.Greet {
        io.WriteString(w, "hello, ")
        io.WriteString(w, html.EscapeString(fmt.Sprint(r.Name)))
        io.WriteString(w, "!")
    } else {
        io.WriteString(w, "goodbye, ")
        io.WriteString(w, html.EscapeString(fmt.Sprint(r.Name)))
        io.WriteString(w, "!")
    }
}

Printing unescaped HTML

The 
 block will print your text as escaped HTML, however, sometimes you need the raw text such as when you're writing JSON.
To do this, simply wrap your Go expression with 
 and 
%>
 tags.

Components

Simple code and print tags work well for simple templates but it can be difficult to make reusable functionality.
You can use the component syntax to print types that implement this 
Renderer
 interface:
type Renderer interface {
    Render(context.Context, io.Writer)
}

Component syntax look likes HTML.
You specify the type you want to instantiate as the node name and then use attributes to assign values to fields.
The body of your component will be assigned as a closure to a field called 
Yield
 on your component type.

For example, let's say you want to make a reusable button that outputs Bootstrap 4.0 code:
We can write this component as an ego template or in pure Go code.
Here we'll write the component in Go:
package myapp

import (
    "context"
    "io"
)
type Button struct {
    Style string
    Yield func()
}
func (r *Button) Render(ctx context.Context, w io.Writer) {
    fmt.Fprintf(w, <div class="btn btn-%s">, r.Style)
    if r.Yield {
        r.Yield()
    }
    fmt.Fprintf(w, </div>)
}

Now we can use that component from a template in the same package like this:
    

        Don't click me!
    


Our template automatically convert our component syntax into an instance and invocation of 
Button
:
var EGO Button
EGO.Style = "danger"
EGO.Yield = func() { io.WriteString(w, "Don't click me!") }
EGO.Render(ctx, w)

Field values can be specified as any Go expression.
For example, you could specify a function to return a value for 
Button.Style
:
Don't click me!

Named closures

The 
Yield
 is a special instance of a closure, however, you can also specify named closures using the 
::
 syntax.

Given a component type:
type MyView struct {
    Header func()
    Yield  func()
}

We can specify the separate closures like this:
    <:header>
        This content will go in the Header closure.
    

This content will go in the Yield closure.


Importing components from other packages

You can import components from other packages by using a namespace that matches the package name
The 
ego
 namespace is reserved to import types in the current package.

For example, you can import components from a library such as bootstrap-ego:
    
        
            

                Don't click me!
            
        
    


Caveats

Unlike other runtime-based templating languages, ego does not support ad hoc templates. All templates must be generated before compile time.

Ego does not attempt to provide any security around the templates. Just like regular Go code, the security model is up to you.