Why are there two assignment operators, `<-` and `->` in R?

Question

I know how to use <- and ->, and there are several writeups on the difference between equals assignment & arrow assignment,

Answer 1

I can’t speculate on R’s reasons for allowing left-to-right assignment. And it’s certainly true that most programming languages (nearly all, in fact) perform only right-to-left assignment.

That said, R isn’t entirely on its own.

I'm not familiar with any other language that allows the right-assignment semantics.

I can think of at least three other (families of) languages that allow it:

• Assembly languages often perform left-to-right assignment; for instance, the influential AT&T syntax writes assignment like this:

  movl $1, %eax
  

  This assigns the value 1 to the EAX register. (On the other hand, Intel’s x86 syntax performs right-to-left assignment.)

• TI-BASIC’s STO (“store”) operation is written like this:

  1→A
  

• COBOL uses multiple forms of left-to-right assignment:

  MOVE 1 TO x
  ADD 2 TO x
  

  etc.

I’m however doubtful whether any of these languages served as inspiration for R’s assignment syntax. By contrast, the APL programming language uses arrow-assignment, and it’s generally accepted that S (and thus indirectly R) took inspiration from that; but APL only performs right-to-left assignment (var ← value).