I see the same thing, but I think it's supposed to be a rightward arrow, as in a function from positive numbers to negative numbers. At least, that's how it usually appears in the Racket documentation.

It does appear to be some encoding issue relating to the Unicode "→" (U+2192). Encoded with UTF-8, this is represented by the three bytes 0xE2 0x86 0x92. In the TeX EC encoding, these correspond to "âĘŠ".

(See https://www.tug.org/TUGboat/Articles/tb11-4/tb30ferguson.pdf, also section 2.3 of http://mirrors.ctan.org/macros/latex/doc/encguide.pdf)

I've submitted a replacement to arxiv that fixes the encoding issues (arxiv unfortunately does not support xelatex, which I use extensively). That will appear Tue 0:00 GMT.

In the meantime, I've made a version available at https://www.cs.umd.edu/~dvanhorn/jfp-draft-2016.pdf

This is now fixed.