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| Forth |
 Title text: I NOTATION POLISH REVERSE ❤️ |
Explanation
Forth is a stack-oriented programming language. Like most stack-oriented programming languages, Forth's grammar incorporates Reverse Polish notation. For example, instead of an expression that adds two numbers via infix notation, such as 8 + 5, Forth would denote it as 8 5 +, indicating that the value "8" is pushed onto the stack, then a value of "5", then the operation "+" will take these latest two items from the stack and push their sum (the value 13) back onto it. Reverse Polish notation is not just limited to mathematical expressions in the language; control flow and function calls also analogously follow this notation.
This is similar to subject–object–verb word order, where a statement in English using subject–verb–object order, as in "Sam ate apples", can be represented as "Sam apples ate".
The comic features a car with a bumper sticker that mentions Forth, but in a way where the message reflects its grammar in a self-referential joke. The bumper sticker is a take on a common type with the format "I ❤️ [object]", where the object is what the driver publicly expresses personal attachment to. The heart symbol is an abbreviation of the word "love". A real-life example is the abbreviated form of the slogan "I Love New York", expressed as "I ❤️ NY". In this case, instead of the expected "I ❤️ FORTH", the sticker reads "I FORTH ❤️" to reference Forth's stack-based logic: interpreting "❤️" as a "word" that takes two arguments, SUBJECT OBJECT, and returns the statement "SUBJECT love(s) OBJECT".
While probably coincidental, as xkcd primarily has body text in all caps anyway, the capitalization of the word "FORTH" also parallels the convention of how the language was stylized in its early years until the late 1970s. This sometimes causes it to mistakenly be assumed to be an acronym.
Not all Forth operators take exactly two arguments. For example, NEGATE returns the negative of the previous argument, such that 1 NEGATE results in -1. In the title text, this logic is applied to adjectives, so that the English word order "adjective–noun phrase" becomes the head-initial "noun phrase–adjective". This puts the two adjectives POLISH and REVERSE after the noun NOTATION, resulting in "I NOTATION POLISH REVERSE ❤️" instead of the original sentence: I love Reverse Polish Notation.
1033: Formal Logic also features a bumper sticker on a car, which involves personal expression. Reverse Polish notation is also the subject of 645: RPS.
Transcript
- [The right rear portion of a car is shown. On the bumper is a sticker that says:]
- I FORTH ❤️
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