template

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Published: Oct 1, 2018 License: Apache-2.0 Imports: 7 Imported by: 0

README

irgen templates

This document describes briefly how irgen templates work.

This place is not a place of honor

I (DE) didn’t want to develop a new template system, and neither should you. Please don’t let this code turn into a nuclear waste dump.

My goal was a simple template that can be compiled as is, to reduce the headache of debugging generated code. I couldn’t find an existing template system that meets this requirement.

Note

This document is a literate Go program. To extract the Go samples from this document:

sed -ne '/^```.*go/,/^```/{s/^```.*//
p
}' README.md >README.go

Here is the program header.

package main

import (
    "bytes"
    "fmt"
    "os"

    "github.com/cockroachdb/cockroach/pkg/sql/ir/irgen/template"
)

Motivation

Let’s suppose that we need to generate structs with a constructor that validates its arguments. The desired output is something like:

type S1 struct {
    a int
}

func MakeS1(
    a int,
) S1 {
    if a < 0 {
        panic("invalid argument a")
    }
    return S1{
        a,
    }
}

type S2 struct {
    a int
    b int
}

func MakeS2(
    a int,
    b int,
) S2 {
    if a < 0 {
        panic("invalid argument a")
    }
    if b < 0 {
        panic("invalid argument b")
    }
    return S2{
        a,
        b,
    }
}

Let’s assume that the input is stored in slice of these structs.

type Struct struct {
    Name   string
    Fields []string
}

The input to generate the code above would be:

var input = []Struct{{"S1", []string{"a"}}, {"S2", []string{"a", "b"}}}
Naive generation

We might try to generate code like this.

func GenerateNaive(structs []Struct) string {
    var b bytes.Buffer
    for _, s := range structs {
        fmt.Fprintf(&b, "type %s struct {\n", s.Name)
        for _, f := range s.Fields {
            fmt.Fprintf(&b, "\t%s int\n", f)
        }
        fmt.Fprintf(&b, "}\n\n")
        fmt.Fprintf(&b, "func Make%s(\n", s.Name)
        for _, f := range s.Fields {
            fmt.Fprintf(&b, "\t%s int,\n", f)
        }
        fmt.Fprintf(&b, ") %s {\n", s.Name)
        for _, f := range s.Fields {
            fmt.Fprintf(&b, "\tif %s < 0 {\n", f)
            fmt.Fprintf(&b, "\t\tpanic(\"invalid argument %s\")\n", f)
            fmt.Fprintf(&b, "\t}\n")
        }
        fmt.Fprintf(&b, "\treturn %s{\n", s.Name)
        for _, f := range s.Fields {
            fmt.Fprintf(&b, "\t\t%s,\n", f)
        }
        fmt.Fprintf(&b, "\t}\n")
        fmt.Fprintf(&b, "}\n\n")
    }
    return b.String()
}

The first version of irgen code generation was written in this style and rejected because it is not easy to change.

Generation with an irgen template

For the irgen template, we write an example of each code block and annotate it with directives indicating repetition.

// @for struct

type StructName struct {
    // @for field
    Field int
    // @done field
}

func MakeStructName(
    // @for field
    Field int,
    // @done field
) StructName {
    // @for field
    if Field < 0 {
        panic("invalid argument Field")
    }
    // @done field
    return StructName{
        // @for field
        Field,
        // @done field
    }
}

// @done struct

Here is the code to instantiate the template (paste the code above to stdin).

func GenerateIrgen(structs []Struct) string {
    root := template.NewRoot()
    for _, s := range structs {
        node := root.NewChild("struct")
        node.AddReplacement("StructName", s.Name)
        for _, f := range s.Fields {
            child := node.NewChild("field")
            child.AddReplacement("Field", f)
        }
    }
    var b bytes.Buffer
    tmpl, _ := template.Parse(os.Stdin)
    tmpl.Instantiate(&b, root)
    return b.String()
}

Additional features

In addition to @for label/@done label, there is @if label/@fi label. The block is processed either zero or one times depending on whether there exists a child node with the specified label.

The first argument to AddReplacement is actually a regexp. The first and second arguments are passed to ReplaceAll in package regexp.

The second argument AddReplacement is an empty interface that is converted to a string using a %s conversion. For other formats and/or arbitrary argument counts, use AddReplacementf.

Replacements are processed in reverse order so that macro-style substitutions are convenient.

Coda

Here is the main function.

func main() {
    fmt.Println(GenerateNaive(input))
    fmt.Println(GenerateIrgen(input))
}

Documentation

Index

Constants

This section is empty.

Variables

This section is empty.

Functions

This section is empty.

Types

type Node

type Node struct {
	// contains filtered or unexported fields
}

Node is a type for hierarchical data.

func NewRoot

func NewRoot() *Node

NewRoot returns a new root node.

func (*Node) AddReplacement

func (node *Node) AddReplacement(expr string, repl interface{})

AddReplacement adds a regexp replacement to the given node. The replacement value is formatted with a %s conversion.

func (*Node) AddReplacementf

func (node *Node) AddReplacementf(expr string, replFormat string, a ...interface{})

AddReplacementf adds a regexp replacement to the given node. The replacement format and arguments are formatted with fmt.Sprintf.

func (*Node) NewChild

func (node *Node) NewChild(label string) *Node

NewChild adds a child with the given label to the given node.

type Template

type Template struct {
	// contains filtered or unexported fields
}

Template is the type of a simple program whose input is hierarchical data (Node) and whose output is text.

func Parse

func Parse(r io.Reader) (*Template, error)

Parse parses a template.

func (Template) Dump

func (tmpl Template) Dump(w io.Writer)

Dump prints the given template to w.

func (*Template) Instantiate

func (tmpl *Template) Instantiate(w io.Writer, node *Node) error

Instantiate executes the given template on the given input.

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