Comment author: @xavierleroy

I agree these examples are surprising, but they are valid OCaml syntax. "a - b" means subtraction, but "- a" means "opposite of a". Hence, "a - - b" means "a - (- b)". Likewise, "a + b" means addition, but "+ a" means "the same value as a". (This is a dubious feature that OCaml inherited from other languages such as C, C++ and Java.) Hence "a + + b" means "a + (+ b)", i.e. "a + b". Hope this explains the mystery.

Comment author: jessicah

They don't work with the preprocessor though, so there's still a bug somewhere...

cat test.ml:
1 + + 2;;
ocamlopt -pp camlp4o test.ml:
File "test.ml", line 1, characters 2-3:
Parse error: [expr] expected after [infix operator (level 2) (start with '+', '-')] (in [expr])
File "test.ml", line 1:
Error: Error while running external preprocessor
Command line: camlp4o 'test.ml' > /tmp/ocamlppd45529