Making a Weather Telegram bot in Purescript

Justin Woo • Software Engineer

In this post, we'll write a Telegram bot to report the weather and go over a wide breadth of features that should get you acquainted with Purescript and some of its libraries. Hopefully this will whet your appetite for writing some Purescript and become more familiar with how pure functional programming looks and works.

Writing the Code

Our bot will need to do five things:

  • Read a config file
  • Create a bot instance
  • Be able to receive messages and use regex to match them
  • Fetch weather data
  • Send the weather data as a response

We'll go over these one by one and put our whole program together.

Reading config

While this doesn't sound glorious, this in itself introduces multiple things we'll need to get into.


Reading a file in a Node.js environment is asynchronous, meaning that we need some way to pass in a function to continue running our program. While we could do this with callbacks, let's use a cleaner way to do this so that we can use more powerful, general techniques. The library normally used for such single-value async in Purescript is purescript-aff, which provides some utilities for running our program as an asynchronous one, and allows us to use async using Affs freely within.

    main = launchAff do
      config <- parseConfig <$> readTextFile UTF8 "config.json"

So here, launchAff lets us, well, launch some Affs, and this first line runs readTextFile from purescript-node-fs-aff, which is an Aff, in order to get our config out. But we've probably already seen some things that are unfamiliar, so let's go over them.

The <$> operator is an alias for map, which you may know from Array#map in Javascript and such. In Purescript, there is a general interface for types that can be mapped to, which is generally defined as a function that takes a function for transforming what is inside and retaining the container, such that for an Array, it maps the function into the elements of the array, and for Aff, it maps the function into the resolved value of the Aff.

So after we read the file, the result inside of our Aff is mapped to a function that will parse the config file contents.

Parsing our config

Our config.json file will look something like this:

      "token": "123456:ABCDEFGHI", // the token you get from the Telegram bot API
      "location": "Helsinki, FI" // the location we'll be getting weather for

And so we can represent it with a Record type (like Objects in Javascript) like so:

    type Token = String -- just a more descriptive type alias
    type Config =
      { token :: Token
      , location :: String

And so with this, we can then parse our JSON and start reading properties from it using purescript-foreign.

    parseConfig :: String -> Either ForeignError Config
    parseConfig string = do
      json <- parseJSON string
      token <- readProp "token" json
      location <- readProp "location" json
        { token
        , location

The type signature for this function is Either ForeignError Config, because while parsing our foreign JSON object, we can end up with all kinds of errors. If the string we pass in is an invalid JSON blob, then this will error out, and the same will happen if one of the properties we try to read either don't match the type we provide or don't exist. Likewise, parseJSON and readProp have types of Either ForeignError _ because each of those operations can fail and immediately return the appropriate ForeignError.

The pure at the end wraps our Config value into the Either, which in this case makes it Right Config to represent a successful parsing operation. pure is like Array.of or Promise.resolve where it wraps the value into a container.

And so with this, we have our Aff (Either ForeignError Config) value which is then resolved using our do-block binding in our Aff "context".

    main = launchAff do
      config <- parseConfig <$> readTextFile UTF8 "config.json"
      case config of
        Right {token, location} -> do
        Left err -> do
          error $ "config.json is malformed: " <> (show err)

Creating a bot instance

Using purescript-node-telegram-bot-api, we get the function connect with type Token -> Eff (TelegramEffects e) Bot, but that's not exactly the type we need, as we need to run this function in the Aff context. Luckily for us, there's a function for this kind of need: liftEff. This allows us to "lift" our function of Eff into an Aff:

    connect' :: forall e.
      String ->
      Aff (telegram :: TELEGRAM | e) Bot
    connect' = liftEff <<< connect

And then in that block above under the Right Config case, we can get our bot:

    case config of
      Right {token, location} -> do
        bot <- connect' token

Receiving messages and matching with regex

In order to handle multiple messages over time, we're going to have to use some kind of multi-value async type. For this, we can use purescript-xstream, which is a layer over the xstream library that Andre wrote.

Like before, this library provides a create function for creating a Stream using an Eff, so we can liftEff to make this an Aff.

    addMessagesListener' :: forall e.
      Bot ->
        ( stream :: STREAM
        , telegram :: TELEGRAM
        , console :: CONSOLE
        | e
        (Stream Int)
    addMessagesListener' bot = liftEff do
        { start: \listener -> do
            addMessagesListener bot getRegex \message string -> do
              let sender =
              log $ "Request from " <> (show sender)
        , stop: const $ pure unit

This passes a Listener, which is a Record of start and stop functions, to the create function. This start function will be called when a listener is added to the stream, when the addMessagesListener will be called on the bot, a regex pattern, and a callback that will send the sender's id number to the listener. When the listener is removed, the stop will be called, which in this will do nothing by always returning pure unit.

The regex pattern used here is prepared in such a fashion:

    getRegex :: Regex
    getRegex = unsafePartial $ fromRight $ regex "^get$" (noFlags {ignoreCase = true})

Constructing regex is an operation which can fail when an invalid pattern is passed in. This code only extracts the successful value. Because the pattern used is only valid for a subset of all possible string values, the value returned is given a Partial constraint. This constraint is unsafely removed, which means that our program will simply crash when an invalid pattern is used (normally resulting in the Left). But if this can crash our program, of course we need to do some work to make sure it doesn't crash.

Using purescript-test-unit, we can test our regex pattern such that it doesn't crash our program and that it does match the values we want to match. Thankfully, this is fairly straightforward:

    testPattern pattern string =
      equal true $ Regex.test pattern string
      main = runTest do
        suite "getRegex" do
          test "does not crash the program" do
            let testPattern' = testPattern getRegex
            testPattern' "get"
          test "tests get 'get' but with case insensitivity" do
            let testPattern' = testPattern getRegex
            testPattern' "get"
            testPattern' "Get"
            testPattern' "GET"
            expectFailure "aget shouldn't work" $ testPattern' "aget"
            expectFailure "geta shouldn't work" $ testPattern' "geta"

Fetch weather data

To fetch our weather data, we'll use the Yahoo Weather API as it's quite easy to use and doesn't require any registration. The gist of this comes down to the type signatures:

    type Result =
      { id :: Int
      , weather :: Either Error Weather
    getWeather :: forall e.
      String ->
      Int ->
        ( stream :: STREAM
        , ref :: REF
        , ajax :: AJAX
        | e
        (Stream Result)

It's an Eff function that will return a Stream of Result, where the weather property data we fetch may end up in an error (if we can't make a successful request or parse the response). Then the rest follows:

    getWeather location id = fromAff do
      attempt' $ do
        let url = "*%20from%20weather.forecast%20where%20woeid%20in%20(select%20woeid%20from%20geo.places(1)%20where%20text%3D%22"
                  <> encodeURI location
                  <> "%22)%20and%20u=%27c%27&format=json"
        response <- get url
        pure $ extractForecast response
        result weather =
            { id
            , weather
        attempt' aff = do
          outcome <- attempt aff
          case outcome of
            Left e ->
              result $ Left e
            Right (Left e) ->
              result (Left $ EffE.error (show e))
            Right (Right weather) ->
              result (Right weather)

We build a url and make a GET request, for which we extract the forecast data and wrap it in a Stream. The main body operates on just the Right Weather case, and errors will be spit out into Left _ if there is a problem.

purescript-aff provides us an attempt function which lets us attempt running Affs. We use this to run our Aff and handle the result as needed to make it match our Result type.

Our JSON parsing code is similar to the code above for how we parsed our config:

    extractForecast :: forall t.
      { response :: String | t }
      -> Either ForeignError Weather
    extractForecast response = do
      json <- parseJSON response.response
      channel <-
        prop "query" >=>
        prop "results" >=>
        prop "channel" $
      city <- prop "location" >=> readProp "city" $ channel
      item <- readProp "item" channel
      condition <- readProp "condition" item
      temp <- readProp "temp" condition
      description <- readProp "text" condition
      forecast <- readProp "forecast" item
      forecastToday <- index 0 forecast
      high <- readProp "high" forecastToday
      low <- readProp "low" forecastToday
        { city
        , temp
        , description
        , high
        , low

The >=> operator is just an alias for "composeKleisli", which just means that we will apply this operation in succession on the argument supplied, which lets us chain operations that take an argument. It looks scary, but is really just a shorter way to write bindings in succession without having to have explicit bindings.

To use our getWeather function, we need to use a bind/flatMap operation to do so. xstream provides a bindEff function, which we can fit to meet our uses like so:

    bindEff' :: forall e a b.
      Stream a ->
      (a -> Eff (stream :: STREAM | e) (Stream b)) ->
      Aff (stream :: STREAM | e) (Stream b)
    bindEff' s p = liftEff $ s `bindEff` p

So just like the other cases where we lifted Eff into Aff. As the signature says, the projecting function passed in will produce a Stream b, and the output will be Stream b. This is because bind/flatMap operations will "concatenate" values emitted by the "inner" stream into our stream. And so we can get our results stream:

    results <- requests `bindEff'` (getWeather location)

Send the weather data as a response

To then add a listener to our stream and make the stream run to actually add a message listener and such, we use the addListener function from xstream the same way as before:

    addListener' :: forall e a.
      Listener e a ->
      Stream a ->
      Aff (stream :: STREAM | e) Unit
    addListener' listener stream = liftEff do
      addListener listener stream

Which is then used like so:

      { next: sendMessage' bot
      , error: message >>> log
      , complete: const $ pure unit

Like before, error composes a function to extract messages from errors with a logging function, and we do nothing on complete. Our next handler has the interesting bit in which we actually prepare messages and send them. We partially apply our bot instance here so that the result can be sent to it correctly. Then our sendMessage' definition sends errors as-is and formats the weather to send to the user:

    sendMessage' :: forall e.
      Bot ->
      Result ->
        ( telegram :: TELEGRAM
        , console :: CONSOLE
        | e
    sendMessage' bot {id, weather} =
      case weather of
        Left e -> do
          let message = show e
          log message
          send message
        Right weather' -> do
          let message = "It's now " <> weather'.temp <> " degrees and " <>
                        weather'.description <> " in " <> weather'.city <> ".\n" <>
                        "Today's high/low is " <> weather'.high <> "/" <> weather'.low <> "."
          log message
          send message
        send = sendMessage bot id


Now we have a Telegram bot that will respond to any variation of "get" with Weather information for Helsinki! We've covered a wide range of features and tools that Purescript and its ecosystem come with, and I hope this has piqued your interest in writing some Purescript for any kind of application you want to make, whether it's also a Telegram bot, a general Node.js application, a front-end web application, or something on any environment that runs Javascript.

You can find the code for this post here: