Query in v1

ichiban/prolog is a Go library and my experiment in placing Prolog into the Go ecosystem.

By placement, I don’t just mean it’s a Prolog implementation written in Go or a Prolog implementation that you can access from Go. I mean it should feel as if it had always been part of Go.

For querying, I modeled the API after database/sql, Go’s standard database interface in the standard library.

sols, err := p.Query(`mortal(Who).`)
if err != nil {
	panic(err)
}
defer sols.Close()

for sols.Next() {
	var s struct {
		Who string
	}
	if err := sols.Scan(&s); err != nil {
		panic(err)
	}

	fmt.Printf("Who = %s\n", s.Who) // ==> Who = socrates
}
if err := sols.Err(); err != nil {
	panic(err)
}

This is almost identical to SQL querying in Go:

rows, err := db.Query(`SELECT * FROM mortals`)
if err != nil {
	panic(err)
}
defer rows.Close()

for rows.Next() {
	var s struct {
		Who string
	}
	if err := rows.Scan(&s.Who); err != nil {
		panic(err)
	}

	fmt.Printf("Who = %s\n", s.Who) // ==> Who = socrates
}
if err := rows.Err(); err != nil {
	panic(err)
}

This makes the API familiar to Go developers and easier to integrate it into their project.

But is the database/sql-like interface perfect for Prolog? There are two things to consider.

First, the database/sql API that I modeled mine after isn’t even considered perfect by Go developers. My Scan() is slightly more powerful because it maps query variable names to the corresponding struct field. This enhancement comes from sqlx, an extension to the standard database/sql. sqlx’s StructScan() maps each column to the corresponding struct field. database/sql is a solid API but very few Go developers use it as is. Most of them use it through a wrapper like sqlx or ORM like GORM

Second, Go has gained new features that might have changed the design decisions. The Go language is slowly but steadily evolving. Go introduced a new standard iterator abstraction iter in Go 1.23, generics in Go 1.18, and generic methods in Go 1.27. These features open up new possibilities for designing an API tailored for Prolog.