Great responses. Thanks! I think what it comes down to is there's two main
definition/use cases of an interface:
- The inheritance-based one that you find in Java and C# where the
interface provides an contract into various implementations. The design
is typically API-driven and you would approach the problem by defining the
interface first. It's usually defined in its own package.
- The other is when you're writing code that depends on an external
dependency and you want to hide it behind an interface. This was not
practical in Java / C# because of the inheritance-based interface, and
showcases the power of Go interfaces. It's usually defined in the package
that needs it.
Both use cases are valid depending on what you're building and both are
possible in Go.
On Monday, November 21, 2016 at 10:30:21 AM UTC-5, Vorn Mom wrote:
>
> Sorry if it was asked before, but where should interfaces live?
>
> - In the package that contains an implementation of it.
> - In its own package.
> - or in the package that needs it.
>
> The standard library is not consistent. For example:
>
> - io.Writer is defined in a package that also has an implementation,
> io.PipeWriter.
> - Encoding is all interfaces, defering implementation to sub-packages.
> - database/sql returns a DB struct on Open() instead of an interface.
>
> Because Go interfaces are decoupled from implementation, I think it should
> be defined where it's needed. So I like having my own interface for DB
> that may only define a subset of what behaviors it provides.
>
> However, doesn't defining an interface that depends on other packages
> decrease cohesion and leaks the abstraction? For example,
>
> type interface DbQuery {
> QueryRow(query string, args ...interface{}) *sql.Row
> }
>
> This interface depends on sql.Row. This decreases cohesion because the
> interface is now defined across packages?
>
> -Vorn
>
>
>
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