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Patterns are half-formed code

·4 mins

If “technology is stuff that doesn’t work yet”1, then patterns are code we don’t know how to write yet.

In the Go Programming Language, the authors show how to iterate over elements in a map, sorted by keys:

To enumerate the key/value pairs in order, we must sort the keys explicitly, for instances, using the Strings function from the sort package if the keys are strings. This is a common pattern.

— Go Programming Language, Alan A. A. Donovan & Brian W. Kernighan, p94

The pattern is illustrated by the following code:

import "sort"

var names []string
for name := range ages {
    name = append(names, name)
for _, name := range names {
    fmt.Printf("%s\t%d\n", name, ages[name])

Peter Norvig calls this an informal design pattern: something referred to by name (“iterate through items in a map in order of keys”) and re-implemented from scratch each time it’s needed.

Informal patterns have their place but they are a larval form of knowledge, stuck halfway between intuition and formal understanding. When we see a recognize a pattern, our next step should always be to ask, “can we make it go away?”

Patterns are one way of expressing “how to” knowledge2 but we have another, better way: code. Source code is a formal expression of “how to” knowledge that we can execute, test, manipulate, verify, compose, and re-use. Encoding “how to” knowledge is largely what programming is 3. We talk about replacing people with programs precisely because we take the knowledge about how to do their job and encode it such that even a machine can understand it.

So how can we encode the knowledge of iterating through the items in a map in order of keys? How can we replace this pattern with code?

We can start by following Peter Norvig’s example and reach for a dynamic programming language, such as Python:

names = []
for name in ages:
for name in names:
    print("{}\t{}".format(name, ages[name]))

This is a very literal translation of the first snippet. A more idiomatic approach would look like:

names = sorted(ages.keys())
for name in names:
    print("{}\t{}".format(name, ages[name])

To turn this into a formal pattern, we need to extract a function that takes a map and returns a list of pairs of (key, value) in sorted order, like so:

def sorted_items(d):
    result = []
    sorted_keys = sorted(d.keys())
    for k in sorted_keys:
        result.append((k, d[k]))
    return result

for name, age in sorted_items(ages):
    print("{}\t{}".format(name, age))

The pattern has become a function. Instead of a name or a description, it has an identifier, a True Name that gives us power over the thing. When we invoke it we don’t need to comment our code to indicate that we are using a pattern because the name sorted_items makes it clear. If we choose, we can test it, optimize it, or perhaps even prove its correctness.

If we figure out a better way of doing it, such as:

def sorted_items(d):
    return [(k, d[k]) for k in sorted(d.keys())]

Then we only have to change one place.

And if we are willing to tolerate a slight change in behavior,

def sorted_items(d):
    return sorted(d.items())

Then we might not need the function at all.

It was being able to write code like this that drew me towards Python and away from Java, way back in 2001. It wasn’t just that I could get more done in fewer lines—although that helped—it was that I could write what I meant.

Of course, these days I’d much rather write:

   import Data.List (sort)
   import qualified Data.HashMap as Map

   sortedItems :: (Ord k, Ord v) => Map.Map k v -> [(k, v)]
   sortedItems d = sort (Map.toList d)

But that’s another story.

Thanks to Tom Hunger for reviewing this. Infelicities are my own.

  1. Bran Ferren, via Douglas Adams ↩︎

  2. Patterns can also contain “when to”, “why to”, “why not to”, and “how much” knowledge, but they always contain “how to” knowledge. ↩︎

  3. The excellent SICP lectures open with the insight that what we call “computer science” might be the very beginning of a science of “how to” knowledge. ↩︎