Predicate Pattern in Go

2025-07-29

Go

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

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

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

Now, for example, we will write a method findProcess for ProcManager on Go with predicate pattern. The ProcManager have field processes []*Process this is slice of processes. And Process have many unique field, that we can use to search. Our method will be get predicate and use this predicate to all items in list, while predicate doesn’t return true or list is end and search return hmm... nothig was found.

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

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

But here we need to handle write a predicate, because this is a more general function. For our case we add intermediate step: predicate builder. Finally we will got this scheme: 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 got one argument: searchPredicate with specific type ProcessSearchPreciate:

type ProcessSearchPredicate func(p *Process) bool

Let’s see: it’s function from Process to bool – 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’s will work like this:

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

This already good, but I want to use autocomplete in my code editor for filter options. For make it we can rewrite filter options from functions to methods and use one empty object for 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 code editor very comfortable:

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