Add an OPAM file for eff.

[yallop]

This should makes the installation instructions a little simpler. Once you have OPAM installed you can switch to a suitable OCaml compiler (opam switch 4.02.1) and then run

opam pin add eff git@github.com:matijapretnar/eff.git

to download the dependencies and build and install eff.

[andrejbauer]

Surely we do not require 4.02 for eff to compile.

[yallop]

Perhaps not; I was going by the README, which says:

To install Eff, you need a standard Unix-style build environment as well as

1. OCaml, version 4.02 or newer, and

[yallop]

I tried building it with a few OCaml versions going back to 3.12.1, and it worked with all of them, so I've removed the OCaml version constraint.

[andrejbauer]

Excellent, many thanks.

[matijapretnar]

Do you both agree we can switch to OCaml 4.02? I want to get rid of

Warning 3: deprecated: String.set
Use Bytes.set instead.

warnings. Anyone not willing to upgrade their OCaml can use the new online toplevel instead.

[andrejbauer]

I think it's more important to allow compilation with older Ocaml versions than getting rid of a warning like that.

[yallop]

One way to avoid the warning is to use the bytes package (distributed with OCamlfind) that provides an implementation of the bytes module for older OCaml versions.

Alternatively, you could add true: warn(-3) to the bottom of the _tags file to turn off warnings about deprecated features.

[matijapretnar]

Okay. Let's keep things simple then and stick with the warning for now. We'll switch to OCaml 4 as soon as we find something that we would like that 3.12 doesn't support.