Code And Cocktails

Standard Joke

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Arabic-to-Roman number conversion is a common programming exercise, often used as a TDD exercise. It is an exercise on many Exercism.IO1 language tracks including my own track for Common Lisp2.

So how do you solve this in some arbitrary programming language?

Typically people write some code to loop through a mapping of numeric values to roman numeral digits and do repeated decrements while accumulating the roman numeral digits. That is to say if you start with 1970 and your mapping contains 1000 => "M"; 900 => "CM"; 500 = "D" you decrement by 1000 and accumulate "M", then since the resulting number is under 1000 you try the next mapping, decrement by 900 and accumulate "CM" etc.

How do you do this in Common Lisp?

(format t "~@r" 1970)

Yes: Built. In. To. The. Language.3 (In fact using ~@:r will provide output as using the alternate roman numeral format which does not have special cases for numbers like 4. i.e. it will output “IIII” rather than “IV”.)

Common Lisp4 is a language with an ANSI specification5. The specification process went from the early-to-mid 1980’s to the mid-1990s. Its goal was to create a single “common” Lisp collecting the good bits from all the splintered Lisp languages used at the time. I’ve heard that it involved a good amount of “horse-trading” as each interested party tried to get the resulting language to contain their special or idiosyncratic feature. So how did it come to have this odd feature?

With a little digging I have found the real story: It was a joke.

In “The Evolution of Lisp”6 Richard Gabriel explains that in 1974 Guy Steele Jr put roman numeral handling into MacLisp7 as a hack. BASE and IBASE were variables which specified the output and input bases for numbers. Before his hack they were limited to vales between 2 and 36. Steele added the ability for them to be set to ROMAN. Thus if set code like (+ i 1) would evaluate to ii since the expression would have been read in as the equivalent of (+ 1 1) and upon output the value 2 would be represented in roman numerals. (It seems in the original announcement of this ‘feature’ that you could also set BASE and IBASE to CUNEIFORM which would cause your Lisp to become wedged. Oh so witty.)

From that time this feature became a bit of a traditional hack - being ported to ZetaLisp8 as part of FORMAT.

Thus when Common Lisp was being created this feature was included in FORMAT9 as it was a normal feature of that function in the Lisps that Common Lisp was being aggregated from.

Since then this feature also found its way into Clojure10 as part of its cl-format11 function.


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