Predicate Pattern in Go

If you think this code is idiomatic, elegant and beautiful, read this article!

FindProcess(ByTitle(title))
FindProcess(ByPID(pid))

This code is an example of the result of the predicate pattern. This example is synthetic, but here we’re trying to achieve a similar result here. Firstly, let’s figure out the predicate. Predicate is a function, that returns a boolean value — very simple!

Now, for example, we will write a method findProcess for ProcManager on Go with predicate pattern. The ProcManager has a field processes []*Process, which is slice of processes. And the Process has many unique fields, that we can use to searching. Our method will take a predicate and apply it to all items in the list until the predicate returns true, or the list ends and the search returns hmm... nothing was found.

For example, in Python we have a standard function filter, which takes an array of items and a predicate function, like this:

filter([1, 2, 3, 4, 5], lambda n: n % 2 == 0)

But here we need to handle predicates manually, because this is a more general function. For our case we add intermediate step: a predicate builder. Finally we will get this scheme:

schema

Let’s write a method findProcess:

func (pm *ProcManager) findProcess(searchPredicate ProcessSearchPredicate) (*Process, error) {
	pm.mu.RLock()
	defer pm.mu.RUnlock()

	for _, proc := range pm.processes {
		if searchPredicate(proc) {
			return proc, nil
		}
	}
	return nil, fmt.Errorf("no process with this parameter")
}

This method takes one argument: searchPredicate with specific type ProcessSearchPreciate:

type ProcessSearchPredicate func(p *Process) bool

Let’s see: it’s function from Process to bool — a predicate. Then we can write a predicate builder functions, like filter options:

func ByTitle(title string) ProcessSearchPredicate {
	return func(p *Process) bool {
		return p.Title == title
	}
}

func ByPID(pid int) ProcessSearchPredicate {
	return func(p *Process) bool {
		return p.PID == pid
	}
}

It will work like this:

proc, err := pm.findProcess(ByTitle(title))

This is already good, but I want to use autocomplete in my code editor for filter options. To make it we can rewrite filter options from functions to methods and use one empty object to call it, like this:

type ProcessSearchFilter struct{}

var filter = ProcessSearchFilter{}

func (f ProcessSearchFilter) ByTitle(title string) ProcessSearchPredicate {
	return func(p *Process) bool {
		return p.Title == title
	}
}

func (f ProcessSearchFilter) ByPID(pid int) ProcessSearchPredicate {
	return func(p *Process) bool {
		return p.PID == pid
	}
}

After that we can use like this:

proc, err := pm.findProcess(filter.ByTitle(title))

And my workflow in my code editor is very comfortable:

demo

If you like my code editor view you can use my Neovim theme: nothing. And you can use my nvim config, find it in my dotfiles

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