The Postman Rides OCaml - Part 1

#ocaml #postman #twitter

This is a walkthrough on how to setup a project so we can run some http requests against the Twitter API with OCaml. If you already now how to do this, you can skip to the next articles in this series. Have at it!

Setting up the OCaml Postman Twitter project

Download opam if you don't have it. Check the install instructions on the link on the opam home page.
After you have it installed make a director for this project and cd into it: mkdir ocaml-twitter-postman && cd ocaml-twitter-postman.
We need to create a compiler specific to this project which we will do in our root directory which is the /ocaml-twitter-postman dir, by running opam switch create . ocaml-base-compiler.4.08.1. This part takes a while so go for a walk around the block. This switch is the one we will use to compile our OCaml code in all the examples. After the process finishes lets do what the output is telling us and run eval $(opam env) to update the current shell environment so it knows to use this compiler.
We are going to need dune to run some commands so let's install it with: opam install dune.
Now let's make a directory for first example, mkdir hello-postman && cd !$ then create a dune file and initiate dune. The dune install command we run below creates a dune-project file we wont ever look at again. Run:

touch dune && dune install

# running `tree` in your terminal gets you:
.
├── _build
│   └── log
├── dune
└── dune-project

1 directory, 3 files

Check out dune.build to get started there if you don't know that its not just a movie.

Create a hello_postman.ml file in the root with the following contents:

Lwt_main.run (Lwt_io.printf "Hello, postman!\n")

Configure the dune file by adding the following to it:

(executable
 (name hello_postman)
 (libraries lwt.unix))

Lets add a Makefile to simplify some running commands. Run touch Makefile and the following to it:

INSTALL_ARGS := $(if $(PREFIX),--prefix $(PREFIX),)

default:
  dune build

install:
  dune install $(INSTALL_ARGS)

uninstall:
  dune uninstall $(INSTALL_ARGS)

reinstall: uninstall install

clean:
  dune clean

.PHONY: default install uninstall reinstall clean

I don't remember where I stole this file from but thank you, nonetheless.

To build and run the project run the following commands from the ./hello-postman directory. make will run the script at the top of the make file then the default argument in which is the build command. This will install any dependencies we listed in our dune file then build the project. This is doing the same thing as when we ran dune install above. So if you run make clean it will get rid of all the generated files. You can then just run make and your deps will be installed and dune-project will be created. #NoteToSelf: Its worth remembering that we are using the compiler we created in the root repository.

make clean
make
dune exec ./hello_postman.exe

# output
➜  hello-postman make
dune build
Info: Creating file dune-project with this contents:
| (lang dune 2.1)
➜  hello-postman dune exec ./hello_postman.exe
Hello, postman!