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Namespaces in Whiley

With the upcoming v0.3.10 release of Whiley, the way import statements are interpreted has changed in a fairly significant manner.  The primary purpose of this is to give better support for [[Namespace (computer science)|namespaces]]. The following illustrates what’s changed:

import whiley.lang.Math

bool check(int x, int y):
    return max(x,y) == x

Previously, the above code would compile with function max() being imported from whiley.lang.Math.  In other words, an import statement automatically imports all names from a given module.  However, this gives relatively little control over namespaces and quickly leads to namespace pollution.

Therefore, in the upcoming release of Whiley, the semantics of import statements has been brought more in line with [[Python (programming language)|Python]].  Thus, the above would not compile as is.  Instead, we would need to write:

import whiley.lang.Math

bool check(int x, int y):
    return Math.max(x,y) == x

This all makes sense, and I’m absolutely happy with the choice to do this.  However, as usual, there are some hidden issues I didn’t foresee.

Root Concepts

The first issue with the above change came out from actually writing code using it!  In particular, I was working on my bytecode disassembler benchmark and constructed a module ClassFile as follows:

define ClassFile as ...
define Reader as ...
define Writer as ...

This gives rise to the types ClassFile.Reader and ClassFile.Writer, both of which make sense. But, it also gives rise to the type ClassFile.ClassFile, which frankly is rather cumbersome. That’s because a ClassFile is both a key concept in my design and, coincidently, a namespace as well. Of course, I could prossibly rename ClassFile to be module Class as follows:

define File as ...
define Reader as ...
define Writer as ...

This gives rise to the types Class.Reader, Class.Writer and Class.File. This is better, but I suspect such renaming won’t always fit well with the top-level design of a program. I imagine Python must suffer from this problem as well, so I’ll have to look into it more …

Processes and Messages

Another problem with the above change to import statements, is how it affects process messages.  The following illustrates:

import whiley.io.File

[byte] ::readFile(String filename):
    fr = File.Reader(filename)
    return fr.read()

This all looks fairly sensible, right?  Well, currently, it doesn’t compile.  The reason becomes apparent if we look inside the File module:

package whiley.io

define Reader as process { ... }

Reader ::Reader(String filename):
    return spawn { ... }

[byte] Reader::read():
     ...

What we see is that read() is a message declared on process type File.Reader. In Java terms, read() is a static method which accepts an argument of type File.Reader. And, therein lies the problem. To get our readFile() example to compile we need to write this:

import whiley.io.File

[byte] ::readFile(String filename):
    fr = File.Reader(filename)
    return fr.(File.read)()

Or, alternatively, we could write it as this:

import whiley.io.File
import read from whiley.io.File

[byte] ::readFile(String filename):
    fr = File.Reader(filename)
    return fr.read()

I find this somewhat annoying.  However, it’s not clear how much of a problem it really is.  That’s because, in practice, we’d probably want to define File.Reader as an interface like so:

package whiley.io

define Reader as interface { [byte] read() }

Reader ::Reader(String filename):
    proc = spawn { ... }
    return { this: proc, read: &read }

[byte] Reader::read():
     ...

An interface is a special kind of record with an explicit field this. Then, when we access the field read, this is automatically used as the receiver. With Reader implemented as above, our original incantation of readFile() would actually compile. That’s because, in this case, fr.read() corresponds to an indirect message send, where as before it was a direct message send.

On the whole, I’m not sure what I’m complaining about! Implementing Reader as an interface versus a process is much of a muchness. There is a minor issue of performance as, for a process you get a static method invocation. But, I’m probably just splitting hairs …

6 comments to Namespaces in Whiley

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