Writing a simple cli with Wreq

Starter project

I remember when I started programming in python. I looked for anything I could write as a CLI or automate in some way, so in the spirit of that, I decided to write a bit about doing the same using haskell libraries.

I replaced monosnap functionality (sharing screenshots) with this script awhile back. It’s generically useful and here is the whole of it if you’re only interested in the source.

Also, there’s a deeper dive into Wreq here if you find this cursory intro lacking

Bam! Some code

The important imports:

import Data.Monoid ((<>)) -- just for glueing together Text's
import Network.Wreq       -- the request library
import Control.Lens       -- setting and getting params/headers/etc
import Data.Aeson.Lens    -- same

Since the main thing this script does is upload a photo anonymously to imgur, we’ll start with that function.

The Request

uploadAndReturnUrl :: IO String
uploadAndReturnUrl = do
  imagePath <- parseArgs
  cid <- clientId
  let authHeader = defaults & header "Authorization" .~ ["Client-ID" <> " " <> cid]
  let payload = [ partText "type" "file"
                , partFile "image" imagePath
                ]
                                  
  res <- postWith authHeader "https://api.imgur.com/3/image.json" payload
  let guid = res ^. responseBody . key "data" . key "id" . _String
  return $ "http://imgur.com/" ++ T.unpack guid

Let’s talk about what might be confusing here. The declaration of authHeader; what is that?

-- in this context, simply a merge 
(&) :: a -> (a -> b) -> b

-- build the header to combine with the defaults :: Options
header :: HeaderName -> ([ByteString] -> f [ByteString]) -> Options -> Options

If you wanted to, you could think of this as an equivalent to the ruby code:

defaults = {:params => [], :headers => []}
defaults[:headers].merge({ :Authorization => "Client-ID #{cid}" })

Fortunately, our code using Lens’ is much more fail-safe. (you could read more here or one of the plethora of other lens tutorials online)

The actual post is pretty self explanatory if you know what authHeader and payload are, but here’s the type sig anyway:

postWith :: Postable a => Options -> String -> a -> IO (Response ByteString)

The Postable typeclass refers to our [Part] which we constructed in the payload declaration, nbd.

The Response

res ^. responseBody . key "data" . key "id" . _String

We use ^. to pull out the image id returned from imgur. Unfortunately, lens’ compose left to right, unlike normal functions in haskell so this is responseBody.data.id

The equivalent ruby code might be something like:

response.body[:data][:id]

Again, ours is a bit safer.

Wrap Up

There’s one other piece which grabs the most recent screenshot.

getRecentPath :: IO FilePath
getRecentPath = do
  home <- getHomeDirectory
  d <- return (joinPath [home, "Desktop"])
  files <- globDir1 (compile "Screen Shot*") d
  case headMaybe (reverse (sort files)) of 
    Nothing -> error "no recent screenshots"
    Just a -> return a

This could be improved by returning an Either String FilePath instead of raising an error, but if we stopped and optimized every chance we had in haskell we’d never finish writing what we started.

At the end of all this, we can screen cap and pop over to a terminal:

imgup --screenshot | pbcopy

Now we have an imgur link in our copy buffer which (for me) completely replaces monosnap’s functionality.