A collection of Either examples in PureScript compared with JavaScript

Note: This is Tutorial 5 in the series Make the leap from JavaScript to PureScript . Be sure to read the series introduction where I cover the goals & outline, and the installation, compilation, & running of PureScript. I will be publishing a new tutorial approximately once-per-week. So come back often, there is a lot more to come!
<< Introduction < Tutorial 4 Part 2 | Tutorial 6 > Tutorial 17 >>

Welcome to Tutorial 5 in the series Make the leap from Javascript to PureScript. I hope you’ve enjoyed the learnings thus far. Be sure to read the series Introduction to learn how to install and run PureScript. I borrowed (with permission) this series outline and javascript code samples from the egghead.io course Professor Frisby Introduces Composable Functional JavaScript by Brian Lonsdorf — thank you, Brian! A fundamental assumption is that you have watched his video before tackling the equivalent PureScript abstraction featured in this tutorial. And for this particular tutorial, you should also review his transcript. It has the imperative and FP code examples in Javascript that I used to port to PureScript. Brian covers the featured concepts extremely well, and it’s better that you understand its implementation in the comfort of JavaScript.

If you read something that you feel could be explained better, or a code example that needs refactoring, then please let me know via a comment or send me a pull request on Github. Finally, If you are enjoying this series then please help me to tell others by recommending this article and/or favoring it on social media

PureScript code organization

Each of the examples below shows the FP code snippet in JavaScript, followed by its port to PureScript. In my Github repository, you will find each code snippet in a separate PureScript file; importing ExampleX.purs and calling it from Main.purs. All my utility functions, including chain and fromNullable, and their corresponding FFI are in the Data folder. Example 5 requires reading a JSON file, so I have created example.json and put it in the folder ./src/resources.

A few of the examples simulate receiving JSON objects and values over the wire. If I were doing this in production, I would probably use purescript-argonaut to parse the JSON and transform the objects and values into PureScript Records and basic types respectively. But I want to keep it simple, so I decided to access the JavaScript objects and their name/value pairs using the FFI. Furthermore, I treat them as Foreign types until I print them to the console.

fromNullable and fromEmptyString

When handling Foreign types returned from a Javascript function or, from JSON coming over the wire, there is always the possibility that they may be null or undefined. But in PureScript there are no null or undefined values, so we typically represent this concept by the value Nothing from the Maybe type. Think of Nothing as something like a type-safe null and, for now, we'll save further details on the Maybe type for a later tutorial.

To match Brian’s code examples, I created fromNullable to check whether a Foreign type is null or undefined, returning Either Error Foreign. Similarly, fromString returns Either Error String, depending on whether a string is empty. I am always looking for opportunities to DRY (Don't Repeat Yourself) out my code and found fromNullable and fromString to be very similar. So I abstracted the repetition into a separate function, toEither. This method is another example of the benefits of PureScript's support for polymorphism. Notice how I was able to make Right x a polymorphic type so that it works for both Foreign, String, and other types.

toEither :: forall a. Boolean -> String -> a -> Either Error a toEither predicate errorMsg value = 
if predicate
then Left $ error errorMsg
else Right value
fromEmptyString :: String -> Either Error String fromEmptyString value = toEither (value == "") "empty string" value 
fromNullable :: Foreign -> Either Error Foreign 
fromNullable value =
toEither (isNull value || isUndefined value)
"null or undefined" value

Example 1 — Point-Free style (tacit programming)

In PureScript and other FP languages, you’ll frequently find that, while a type declaration of a function states that it accepts arguments (or points), the actual arguments are missing in the implementation. We call this paradigm, point-free or tacit style programming, and it helps ‘sometimes’ to give a precise definition of the function. I said ‘sometimes’, because point-free can also obscure the meaning of a function; particularly when an argument name helps in understanding a function’s implementation. The PureScript example below has been written in a point-free style.

/* Example 1 - Javascript */
const openSite = () => 
.fold(showLogin, renderPage)

{- Example 1 - Purescript -}
openSite :: Foreign -> String
openSite =
fromNullable >>>
either (\_ -> "showLogin") \_ -> "renderPage"

I chose point-free to illustrate this paradigm and to take advantage of function composition. That is, openSite = fromNullable >>> . . . instead of openSite currentUser = (fromNullable currentUser) # . . .

But it is debatable whether it remains clear that it takes a current user. So, given that your mileage will vary, always proceed with caution when deciding whether to use point-free style.

Example 2

/* Example 2 - JavaScript */
const getPrefs = user => 
(user.premium ? Right(user) : Left('not premium'))
.map(u => u.preferences)
.fold(() => defaultPrefs, prefs => loadPrefs(prefs))

{- Example 2 - PureScript -}
getPrefs :: Foreign -> String
getPrefs user =
toEither (getPremium user) "not premium" user #
map getPreferences >>>
either (\_ -> defaultPrefs) \prefs -> "loadPrefs " <> prefs

Example 3 — Say goodbye to chain

/* Example 3 - JavaScript */
const streetName = user => 
.chain(a => fromNullable(a.street))
.map(s => s.name)
.fold(e => 'no street', n => n)

{- Example 3 - PureScript -}
streetName :: Foreign -> String
streetName user =
(fromNullable $ getAddress user) >>=
(\address -> fromNullable $ getStreet address) >>=
(\street -> fromNullable $ getStreetName street) >>>
map (\name -> unsafeFromForeign name :: String) #
either (\_ -> "no street") id

Wait! What happened to chain? Well, I have a secret that I've been keeping since the last tutorial - you can replace chain with bind (>>=)! I know, shocking isn't it. Perhaps I should have come clean earlier, but I felt it was better to stick with chain from Brian’s tutorials. So why and when can we replace chain with bind you ask? Let's look at their type declarations to see if that helps to illuminate things:

chain :: forall a b e. (a -> Either e b) -> Either e a -> Either e b bind  :: forall a b.   m a               -> (a -> m b) -> m b

Nope, not really. Hold on a minute — let’s try looking at their implementations. First, chain:

chain :: forall a b e. (a -> Either e b) -> Either e a -> Either e b
chain f = either (\e -> Left e) (\a -> f a)

Then, let’s look at the bind class instance forEither from Either.purs:

-- | The `Bind` instance allows sequencing of `Either` values and functions that
-- | return an `Either` by using the `>>=` operator:
-- |
-- | ``` purescript
-- | Left x >>= f = Left x
-- | Right x >>= f = f x
-- | ```
instance bindEither :: Bind (Either e) where
bind = either (\e _ -> Left e) (\a f -> f a)

Interesting, so bind is quite similar to my implementation of chain, but with the first two arguments flipped! Why does this work? Well, if bind sees that the first argument is Left e then, like chain, it ignores the the second function argument (\a f -> f a), and returns the first argument, Left e. But if the first argument is Right a then the second function argument is applied. Finally, becausef a -> Either e b , then Either e bwill be returned.

The next question is ‘when can I substitute chain with bind?’. Well, for all intents and purposes, they’re interchangeable! Consequently, you won't find a class instance of chain for Either in Either.purs. Take a look at Example 6, where I've provided two versions of parseDbUrl - one using chain and the other using bind. That example should help you to go back and refactor any functions using chain to bind. So . . . say goodbye to chain and long live bind!

Example 4 — Anonymous Function Arguments

/* Example 4 - JavaScript */
const concatUniq = (x, ys) =>
fromNullable(ys.filter(y => y === x)[0])
.fold(() => ys.concat(x), y => ys)

{- Example 4 - PureScript -}
concatUniq :: String -> String -> String
concatUniq x ys =
filter (_ == x) ys #
fromEmptyString #
either (\_ -> ys <> x) \_ -> ys

This tip is straightforward and very useful in practice. In the PureScript example above, notice the expression filter (_ == x) ys. You may be wondering why I didn't write it as filter (\y -> y == x) ys. Well, because I'm using an anonymous function argument _, represented in the predicate portion of the filter function. Think of _ as a little syntax sugar that helps to shorten your code. You'll be pleased to know that anonymous function arguments work for Records and other types of expressions, as well. You can learn everything you need to know about them in this blog post, which is part of the series '24-days-of-purescript-2016'.

Example 5 — let vs. where keywords

Before discussing the let and where keywords, let me mention that this code snippet makes good use of native side effects. That topic was well covered in Part 2 of Tutorial 4, so if you're still shaky on File IO and exception handling then go back and have a look. Now onto the use of where vs. let keywords in the duel wrapExample snippets.

/* Example 5 - JavaScript */
const readFile = x => tryCatch(() => fs.readFileSync(x)) 
const wrapExample = example => 
.fold(() => example,
preview => Object.assign({}, example, preview))

{- Example 5 - PureScript -}
wrapExample :: forall eff
. Foreign
-> Eff (fs :: FS, exception :: EXCEPTION | eff) Foreign wrapExample example =
fromNullable (getPreviewPath example) #
map (\path -> unsafeFromForeign path :: String) >>>
either (\_ -> pure example) wrapExample'
wrapExample' pathToFile =
(try $ readTextFile UTF8 pathToFile) >>=
either Left parseValue >>>
either (\_ -> example) (assignObject2 example) >>>
wrapExample_ :: forall eff
. Foreign
-> Eff (fs :: FS, exception :: EXCEPTION | eff) Foreign wrapExample_ example =
fromNullable (getPreviewPath example) #
map (\path -> unsafeFromForeign path :: String) >>>
let wrapExample' pathToFile =
(try $ readTextFile UTF8 pathToFile) >>=
either Left parseValue >>>
either (\_ -> example) (assignObject2 example) >>>
in either (\_ -> pure example) wrapExample'

We have seen the where keyword many times before. It is usually at the top of a module, for the purpose of delineating the block of code represented by the module name. But, so far, I haven't used it inside a function. The purpose is the same - introduce a new block of code, indenting that code so that the compiler understands that where is bound to the syntactic construct (i.e., the new block of code). In the example, you can see that where is forever linked to the code that defines wrapExample'.

Now, take a look at let . At first blush, the purpose of where and let appear to be identical, and this is roughly correct. But there is a subtle difference! let . . . in . . . is also an expression, and therefore can be written wherever expressions are allowed. The code snippet best explains this difference. I used let . . . in . . . as an expression by inserting it between the map and either functions. I could have gone even further, with the following:

map (\path -> unsafeFromForeign path :: String) >>> 
either (\_ -> pure example)
let wrapExample' pathFile = . . .
in wrapExample'

But, then it becomes a matter of readability.

For more information on let vs. where, check out Let vs. Where from wiki.haskell.org. Oh, and you are going to find that the syntax of Haskell is surprisingly similar to PureScript! I've heard a few PureScripters commenting that they were able to pick up Haskell much more quickly, thanks to having learned PureScript first. Likewise, I began learning Haskell first and found I was able to pick up PureScript more rapidly.

Example 6 — Regular expression validators and partial functions

First, before discussing regular expressions in PureScript, my reason for creating the dual code snippets below, parseDbUrl and parseDbUrl_, is to demonstrate how to port functions that use chain, to use bind instead. This code snippet will be the last time you will see chain in my tutorials. And to learn why, then please review the discussion in Example 2.

/* Example 6 - JavaScript */
const parseDbUrl = cfg => 
tryCatch(() => JSON.parse(cfg))
.chain(c => fromNullable(c.url))
.fold(e => null,
u => u.match(/postgres:\/\/([^:]+):([^@]+)@([^:]+):(\d+)\/(.+)/))

{- Example 6 - PureScript -}
dBUrlRegex :: Partial => Regex
dBUrlRegex =
case regex
"^postgres:\\/\\/([a-z]+):([a-z]+)@([a-z]+)\\/([a-z]+)$" noFlags of
Right r -> r
matchUrl :: Regex -> String -> Either Error (Array (Maybe String)) matchUrl r url =
case match r url of
Nothing -> Left $ error "unmatched url"
Just x -> Right x
parseDbUrl_ :: Partial => String -> Array (Maybe String)
parseDbUrl_ =
parseValue >>>
chain (\config -> fromNullable $ getDbUrl config) >>>
map (\url -> unsafeFromForeign url :: String) >>>
chain (matchUrl dBUrlRegex) >>>
either (\_ -> singleton Nothing) id
parseDbUrl :: Partial => String -> Array (Maybe String)
parseDbUrl s =
(parseValue s) >>=
(\config -> fromNullable $ getDbUrl config) >>>
map (\url -> unsafeFromForeign url :: String) >>=
(\r -> matchUrl dBUrlRegex r) #
either (\_ -> singleton Nothing) id

Like JavaScript, PureScript supports the use of regular expressions very well — by wrapping JavaScript’s very own RegExp object! The types and functions are part of the purescript-strings library, located in the module Data.String.Regex. First up is the Regex object.

There’s some new pieces of syntax in theDbUrlRegex function, namely Partial and unsafePartial. In this instance, they allow us to treat a non-exhaustive case expression as a regular case expression (unsafely). So why did I decide on unsafePartial? I tested the regular expression "^postgres:\\/\\/([a-z]+):([a-z]+)@([a-z]+)\\/([a-z]+)$" and I know it works for the sole purpose of demonstration in this tutorial! So no need to bother returning and dealing with an Either Error Regex. You can also take advantage of unsafePartial to return partial functions; again unsafely. And, as a consequence of DbUrlRegex, that is exactly what I am doing in parseDbUrl.

You’ll often hear from functional programmers the phrase, ‘Let the types be your guide’. So, with that mantra in mind, DbUrlRegex is a partial function, so I declare that it belongs to the Partial class (i.e., dbUrlRegex :: Partial => Regex). This fact propagates all the way back to main. I declare that parseDbUrl returns a partial function and, consequently, in the main code snippet below, I use the unsafeFromPartial function to log the result to the console. So, as Eff indicates the presence of side effects, Partial is our way of telling future maintainers of our code that if you change the regular expression, you better be 100% sure that it works.

One final item — parseDbUrl returns Array (Maybe String), and so you're probably wondering about the Maybe constructor. As I mentioned in the fromNullable and fromEmptyString section, we'll cover that abstraction in a future tutorial.

Main program

defaultConfig :: String
defaultConfig = "{ \"url\": \"postgres:\\/\\/username:password@localhost/mydb\"}\n"
main :: Effect Unit
main = do
log "A collection of Either examples"
log "Example 1"
log $ openSite getCurrentUser

log "Example 2"
log $ getPrefs getCurrentUser
  log "Example 3"
log $ streetName getCurrentUser
log "Example 4"
log $ concatUniq "x" "ys"
log $ concatUniq "y" "y"
  log "Example 5"
log "using where keyword in wrapExample"
example <- wrapExample getCurrentExample
log $ unsafeFromForeign example :: String
log "Example 6"
log "Using bind to help parse the database URL"
logShow $ unsafePartial $ parseDbUrl defaultConfig
  log "Game Over"

In the next Tutorial we’re going to learn about Semigroups. Again, If you are enjoying this series then please help me to tell others by recommending this article and/or favoring it on social media. Thank you and till next time!