purefunction

README

purefunction

Given a Go source file, find the names of all functions that are known to be pure.

A "pure function" for this module is, a function that:

• Only calls functions that are known to be pure, if any.
• Does not read or write to any global variables.
• Does not have pointers or slices as function arguments.
• Does not read or write to any memory location using pointers.
• Ideally: Always returns the same answer, given the same input, but this is hard to test for (ref. halting problem).

Examples of pure functions

Example of a pure function:

func add(a, b int) int {
    return a + b
}

Another example of a pure function (even though it is a "method"):

func (c *Config) Add(a, b int) int {
    return a + b
}

Features and limitations

• Functions are filtered out if they have non-pure indicators. The ones that are left are considered pure.
• Functions that read from a file, read from the keyboard, uses randomness or the system time are not pure, but may be detected as such.

Approach

• Uses go/ast extensively.

Memoization

Pure functions, like the fibonacci function, has great potential for optimization by memoization.

Benchmark output for "fibonacci" vs "memoized fibonacci":

goos: linux
goarch: amd64
pkg: github.com/xyproto/purefunction/example
BenchmarkFib10-8                   30586             38894 ns/op
BenchmarkFibMemoized-8          84117813                14.2 ns/op
PASS
ok      github.com/xyproto/purefunction/example 2.798s

See issue #1.

Requirements

• Go 1.10 or later.

General info

• Version: 1.0.1
• License: BSD-3
• Author: Alexander F. Rødseth <xyproto@archlinux.org>

Documentation

Index

• Variables
• func FunctionDecls(filename string) ([]*ast.FuncDecl, error)
• func Pure(funcDecl *ast.FuncDecl, existingPureFuncDecls ...*ast.FuncDecl) bool
• func PureFunctions(filename string) ([]string, error)
• func SetVerbose(verbose bool)

Examples

• PureFunctions

Variables

var Verbose bool

Verbose can be set to true for output to stdout while processing source files

Functions

func FunctionDecls

func FunctionDecls(filename string) ([]*ast.FuncDecl, error)

FunctionDecls returns the pure *ast.FuncDecl functions from the given source file

func Pure

func Pure(funcDecl *ast.FuncDecl, existingPureFuncDecls ...*ast.FuncDecl) bool

Pure examines if the given function is pure or not. No need to pass in existing pure function declarations, it can be skipped.

func PureFunctions

func PureFunctions(filename string) ([]string, error)

PureFunctions returns a slice with the function names that are considered pure

Example

filename := "testdata/main.go"
pureFunctions, err := PureFunctions(filename)
if err != nil {
	log.Fatalln(err)
}
fmt.Printf("Pure functions in %s:\n", filename)
sort.Strings(pureFunctions)
for _, name := range pureFunctions {
	fmt.Println(name)
}

Output:

Pure functions in testdata/main.go:
add
add2
mul
mul3

func SetVerbose

func SetVerbose(verbose bool)

SetVerbose can be used for enabling or disabling verbose output to stdout