mime-types for Io

Series: Seven Languages

Welcome back. I haven’t posted here in five and a half years and the website has been offline for at least two years, but that will be changing, starting now.

I recently finished reading Seven Languages in Seven Weeks. As an exercise, I ported mime-types for Ruby to Io on the 17th of September.

mime-types for Io is both a library and a registry for type information (and reuses the JSON registry from the Ruby library). It works similarly to the Ruby version¹:

plaintext := MimeTypes["text/plain"]
# returns list(text/plain)
text := plaintext first

text mediaType println            # => text
text subType println              # => plain
text extensions join(" ") println # => asc c cc h hh…
text encoding println             # => quoted-printable
text isBinary println             # => false
text isAscii println              # => true
text isObsolete println           # => false
text isRegistered println         # => true
text isLike("text/plain") println # => true
MimeType simplifiedFor("x-appl/x-zip") println
                                  # => "appl/zip"

I’m reasonably happy with the code that I wrote in the port, although I am not sure if it would count as “idiomatic” Io. The most difficult part was implementing foreach. Let’s compare how you do this in Ruby versus Io.

class Collection
  def each
    @inner_collection.each { |v| yield v }
  end
end

This is fairly simple to understand; when you iterate over a Collection registry, you will loop through each item and yield it to the provided block.

The implementation of the equivalent method in Io is a little more complex. The canonical foreach method accepts either two or three parameters, and the first parameter is the optional parameter:

collection foreach(value, message)
collection foreach(index, value, message)

An additional difficulty is that the index and value parameters are names that will be used in the message. Understanding this and implementing it properly is key to understanding Io. It took me a while, but the key is to treat the provided names as slots on the call sender. So, as you’re iterating through an inner collection for which you’re exposing foreach, you set the slot to the iterated value. This is what a two-parameter foreach(value, method) implementation might look like:

foreach := method(
  name := call argAt(0) name
  result := nil

  innerCollection foreach(item,
    call sender setSlot(name, item)
    result := call evalArgAt(1)
  )

  result
)

How does it work? If we follow the execution of foreach(v, v println), we will be working with a slot called v. As we step through innerCollection, we set the call sender’s v slot with the value of item. We then evaluate the message argument (call evalArgAt(1))—which executes in the context of call sender, which is how it can access the v slot.

The next step is to support the optional index parameter. It’s not hard, but it requires modification to the loop in the innerCollection. The main thing to remember is that the message argument may be at offset 1, but if there is an index parameter, it will be at offset 2. (This would be easier if the message were guaranteed to be in a fixed position, but such is life.) So we need to store the message offset in a slot within the function.

foreach := method(
  // Assume value, message by default
  indexName := nil
  valueName := call argAt(0) name
  msgOffset := 1

  if(call argCount == 3, // But we have index, value, message
    indexName := call argAt(0) name
    valueName := call argAt(1) name
    msgOffset := 2
  )

  result := nil

  innerCollection foreach(index, item,
    if(indexName isNil not, call sender setSlot(indexName, index))
    call sender setSlot(valueName, item)
    result := call evalArgAt(msgOffset)
  )

  result
)

Now, if we follow foreach(i, v, list(i, v) println), slots for i and v will be set on the call sender, so that when call evalArgAt(msgOffset) is evaluated, both the index and the value will be available to the message. Because innerCollection is just a simple collection, it is possible to use the foreach(index, value, message) version—but in the implementation for MimeTypes, the collection is a Map and each value is a list—and the iteration is across the items in the contained list, so the index needs to be managed inside of the created foreach loop.

All in all, I found Io interesting to work with and plan on maintaining this library moving forward to keep in practice with Io, even if I don’t end up shipping anything else in Io. I will also port mime-types to Prolog, Scala, Erlang, Clojure, and Haskell—but each of these languages are not as accessible as Io turned out to be, especially as I wish to continue using the infrastructure provided by the Ruby version.