roc/roc-for-elm-programmers.md

Roc for Elm programmers

Roc is a direct descendant of the Elm programming language. The two languages are similar, but not quite the same!

This is a guide to help Elm programmers learn what's different between Elm and Roc.

NOTE: As of 2020, only a subset of what's in this document has been implemented.

Comments

Like Python, Ruby, Perl, and Elixir, inline comments in Roc begin with # instead of --.

Doc comments begin with ## instead.

Like Python, Roc does not have multiline comment syntax.

String Interpolation

Roc strings work like Elm strings except that they support string interpolation. Here's a Roc string which uses interpolation:

"Hi, my name is \(name)!"

The Elm equivalent would be:

"Hi, my name is " ++ name ++ "!"

This interpolation syntax comes from Swift. Only a single identifier can go inside the parentheses (like (name) here), and the identifier needs to be a string already. Arbitrary expressions are not allowed, which means weird situations like string literals inside string literals do not come up.

Roc strings also have the type Str rather than Elm's String. This is to make common qualified operations like Str.toInt more concise; the idea is that you'll use the abbreviation often enough that you'll quickly get used to it. (I got used to str in Rust very quickly.)

Type Aliases

Rather than a type alias keyword, in Roc you define type aliases with : like so:

Username : Str

You can also define type aliases anywhere, not just at the top level. Their scoping rules work as normal.

Separately, Roc also allows standalone type annotations with no corresponding implementation. So I can write this as an annotation with no implementation:

getUsername : User -> Username

Roc will automatically fill in the implementation of this as the equivalent of a Debug.todo. If it ever gets run, it will crash, but for debugging purposes or sketching out APIs, you don't need to bother writing getUsername = Debug.todo "implement".

let syntax

Imagine if Elm's let...in worked exactly the same way, except you removed the let and in keywords. That's how it works in Roc.

For example, this Elm code computes someNumber to be 1234:

someNumber =
    let
        foo =
            1000

        blah =
            234
    in
    foo + blah

Here's the equivalent Roc code:

someNumber =
    foo =
        1000

    blah =
        234

    foo + blah

Like let...in in Elm, this is indentation-sensitive. Each of the definitions ("defs" for short) must have the same indentation as the ending expression.

Function definitions

Roc only has one syntax for defining functions, and it looks almost exactly like Elm's anonymous functions. The one difference is that multiple arguments are separated by commas.

So where in Elm you might write foo a b = in Roc you'd write foo = \a, b -> instead.

One minor benefit of the comma is that you don't need to use parens to destructure arguments inline. For example, in Elm, you always need to use parentheses to destructure variants inline in function declarations, like in these two examples:

\(UserId id1) (UserId id2) ->
\(UserId id) ->

Without the parentheses, it wouldn't be clear where one argument ended and the next one began.

In Roc, the commas make argument boundaries unambiguous, so no parens are needed. You can rewrite the above like so:

\UserId id1, UserId id2 ->
\UserId id ->

Unbound type variables

In Elm, every type variable is named. For example:

List.reverse : List a -> List a

[] : List a

The a in List.reverse is a bound type variable, because it appears more than once in the type. Whatever the first list's a is, that's what the second list's a must be as well.

The a in [] is an unbound type variable. It has no restrictions, which is why [] can be passed to any function that expects a List.

In Roc, this distinction between bound and unbound type variables is reflected at the syntax level. Here are those types in Roc:

List.reverse : List a -> List a

[] : List *

The * is the "wildcard" type variable. It is only for unbound type variables like this. Like the wildcard * in path globs like *.txt, it matches anything.

You can still choose names for unbound type variables if you like, but the compiler will infer them as * by default.

In Elm, the type of always is a -> (b -> a). In Roc, it is:

always : a -> (* -> a)

This makes unbound type variables easier to talk about out loud. Rather than saying (for example) "List a" or "Html msg with a lowercase m" you can say "List star" or "Html star".

Pattern matching

Roc's pattern matching conditionals work about the same as how they do in Elm. Here are two differences:

• Roc uses the syntax when...is instead of case...of
• In Roc, you can use | to handle multiple patterns in the same way

For example:

when color is
    Blue -> 1
    Green | Red | Yellow -> 2
    Purple -> 3

Custom Types

This is the biggest semantic difference between Roc and Elm.

Let's start with the motivation. Suppose I'm using a platform for making a web server, and I want to:

• Read some data from a file
• Send a HTTP request containing some of the data from the file
• Write some data to a file containing some of the data from the HTTP response

Assuming I'm writing this on a Roc platform which has a Task-based API, here's how that code might look:

doStuff = \filename ->
    Task.after (File.read filename) \fileData ->
    Task.after (Http.get (urlFromData fileData)) \response ->
    File.write filename (responseToData response)

A few things to note before getting into how this relates to custom types:

1. This is written in a style designed for chained effects. It's kinda like do notation, but implemented as a formatting convention instead of special syntax.
2. In Elm you'd need to add a <| before the anonymous functions (e.g. <| \response ->) but in Roc you don't. (That parsing design decision was partly motivated by supporting this style of chained effects.)
3. Task.after is Task.andThen with its arguments flipped.

What would the type of the above expression be? Let's say these function calls have the following types:

File.read : Filename -> Task File.Data File.ReadErr
File.write : Filename, File.Data -> Task File.Data File.WriteErr
Http.get : Url -> Task Http.Response Http.Err

after : Task a err, (a -> Task b err) -> Task b err

If these are the types, the result would be a type mismatch. Those Task values have incompatible error types, so after won't be able to chain them together.

This situation is one of the motivations behind Roc's tags feature. Using tags, not only will this type-check, but at the end we get a combined error type which has the union of all the possible errors that could have occurred in this sequence. We can then handle those errors using a single when, like so:

when error is
    # Http.Err possibilities
    PageNotFound -> ...
    Timeout -> ...
    BadPayload -> ...

    # File.ReadErr possibilities
    FileNotFound -> ...
    ReadAcessDenied -> ...
    FileCorrupted -> ...

    # File.WriteErr possibilities
    DirectoryNotFound -> ...
    WriteAcessDenied -> ...
    DiskFull -> ...

Here is the set of slightly different types that will make the original chained expression compile. (after is unchanged.)

File.read : Filename -> Task File.Data (File.ReadErr *)
File.write : Filename, File.Data -> Task File.Data (File.WriteErr *)
Http.get : Url -> Task Http.Response (Http.Err *)

after : Task a err, (a -> Task b err) -> Task b err

The key is that each of the error types is a type alias for a Roc tag union. Here's how they look:

Http.Err a : [ PageNotFound, Timeout, BadPayload ]a
File.ReadErr a : [ FileNotFound, Corrupted, BadFormat ]a
File.WriteErr a : [ FileNotFound, DiskFull ]a

In Elm, these would be defined as custom types (aka algebraic data types) using the type keyword. However, instead of traditional algebraic data types, Roc has only tags - which work more like OCaml's polymorphic variants, and which can be used in type aliases without a separate type keyword. (Roc has no type keyword.)

Here are some examples of using tags in a REPL:

> True
True : [ True ]*

> False
False : [ False ]*

> Ok 5
Ok 5 : [ Ok Int ]*

> SomethingIJustMadeUp 1 2 "stuff"
SomethingIJustMadeUp 1 2 "stuff" : [ SomethingIJustMadeUp Int Int Str ]*

> x = Foo
Foo : [ Foo ]*

> y = Foo "hi" 5
Foo "hi" 5 : [ Foo Str Int ]*

> z = Foo 1 2
Foo 1 2 : [ Foo Int Int ]*

Tags have a lot in common with traditional algebraic data types' variants, but are different in several ways.

One difference is that you can make up any tag you want, on the fly, and use it in any module, without declaring it first. (These cannot be used to create opaque types; we'll discuss those in the next section.)

Another difference is that the same tag can be used with different arities and types. In the REPL above, x, y, and z, can all coexist in the same module even though they use Foo with different arities - and also with different types within the same arity.

Now let's say I do a pattern match with no type annotations.

when foo is
    MyInt num -> num + 1
    MyFloat float -> Num.round float

The inferred type of this expression would be [ MyInt Int, MyFloat Float ], based on its usage.

As with OCaml's polymorphic variants, exhaustiveness checking is still in full effect here. It's based on usage; if any code pathways led to foo being set to the tag Blah, I'd get an exhaustiveness error because this when does not have a Blah branch.

There's an important interaction here between the inferred type of a when-expression and the inferred type of a tag value. Note which types have a * and which do not.

x : [ Foo ]*
x = Foo

y : [ Bar Float ]*
y = Bar 3.14

toInt : [ Foo, Bar Float ] -> Int
toInt = \tag ->
    when tag is
        Foo -> 1
        Bar float -> Num.round float

Each of these type annotations involves a tag union - a collection of tags bracketed by [ and ].

• The type [ Foo, Bar Float ] is a closed tag union.
• The type [ Foo ]* is an open tag union.

You can pass x to toInt because an open tag union is type-compatible with any closed tag union which contains its tags (in this case, the Foo tag). You can also pass y to toInt for the same reason.

In general, when you make a tag value, you'll get an open tag union (with a *). Using when can get you a closed union (a union without a *) but that's not always what happens. Here's a when in which the inferred type is an open tag union:

alwaysFoo : [ Foo Int ]* -> [ Foo Int ]*
alwaysFoo = \tag ->
    when tag is
        Foo num -> Foo (num + 1)
        _ -> Foo 0

The return value is an open tag union because all branches return something tagged with Foo.

The argument is also an open tag union, because this when-expression has a default branch; that argument is compatible with any tag union. This means you can pass the function some totally nonsensical tag, and it will still compile.

Note that the argument does not have the type *. That's because you cannot pass it values of any type; you can only pass it tags!

You could, if you wanted, change the argument's annotation to be []* and it would compile. After all, its default branch means it will accept any tag!

Still, the compiler will infer [ Foo Int ]* based on usage.

Just because [ Foo Int ]* is the inferred type of this argument, doesn't mean you have to accept that much flexibility. You can restrict it by removing the *. For example, if you changed the annotation to this...

alwaysFoo : [ Foo Int, Bar Str ] -> [ Foo Int ]*

...then the function would only accept tags like Foo 5 and Bar "hi". By writing out your own annotations, you can get the same level of restriction you get with traditional algebraic data types (which, after all, come with the requirement that you write out their annotations). Using annotations, you can restrict even when-expressions with default branches to accept only the values you define to be valid.

In fact, if you want a traditional algebraic data type in Roc, you can get about the same functionality by making (and then using) a type alias for a closed tag union. Here's exactly how Result is defined using tags in Roc's standard library:

Result ok err : [ Ok ok, Err err ]

You can also use tags to define recursive data structures, because recursive type aliases are allowed as long as the recursion happens within a tag. For example:

LinkedList a : [ Nil, Cons a (LinkedList a) ]

Inferred recursive tags use the as keyword, which is what OCaml does to display inferred types of recursive polymorphic variants. For example, the inferred version of the above type alias would be:

[ Nil, Cons a b ] as b

The * in open tag unions is actually an unbound ("wildcard") type variable. It can be bound too, with a lowercase letter like any other bound type variable. Here's an example:

incrementFoo : [ Foo Int ]a -> [ Foo Int ]a
incrementFoo = \tag ->
    when tag is
        Foo num -> Foo (num + 1)
        other -> other

The * says "this union can also include any other tags", and here the a says "the return value union includes Foo Int, plus whichever other tags the argument includes in its union."

The Roc type [] is equivalent to Elm's Never. You can never satisfy it!

One final note about tags: tag application is not the same as function application, the way it is with Elm's variants. For example:

• foo bar is function application, because foo is lowercase.
• Foo bar is tag application, because Foo is uppercase.

So this wouldn't compile:

foo : [ Foo ]*
foo = Foo

foo bar

You can't "call" the type [ Foo ]* because it's not a function.

In practical terms, this also means you can't do |> Decode.map UserId because UserId is not a function, and map expects a function. This code would work in Elm, but in Roc you'd need to use an anonymous function - e.g. |> Decode.map (\val -> UserId val) - or a helper function, e.g. |> Decode.map UserId.fromInt

Opaque Types

The tags discussed in the previous section are globally available, which means they cannot be used to create opaque types.

Private tags let you create opaque types. They work just like the global tags from the previous section, except:

• Private tags begin with an @ (e.g. @Foo instead of Foo)
• Private tags are scoped to the current module, rather than globally scoped
• Private tags can only be instantiated in the current module

For example, suppose I define these inside the UserId module:

UserId : [ @UserId Int ]

fromInt : Int -> UserId
fromInt = \int ->
    @UserId int

toInt : UserId -> Int
toInt = \@UserId int ->
    int

I can now expose the UserId type alias, which other modules can use as an opaque type.

It's not even syntactically possible for me to expose the @UserId tag, because @ tags are not allowed in the exposing list. Only code written in this UserId module can instantiate a @UserId instance.

If I were to write @UserId inside another module (e.g. Main), it would compile, but that @UserId would be type-incompatible with one created inside the UserId module. Even trying to use == on them would be a type mismatch, because I would be comparing a [ UserId.@UserId Int ]* with a [ Main.@UserId Int ]*, which are incompatible.

Phantom Types

Phantom types exist in Elm but not in Roc. This is because phantom types can't be defined using type aliases (in fact, there is a custom error message in Elm if you try to do this), and Roc only has type aliases. However, in Roc, you can achieve the same API and runtime performance characteristics as if you had phantom types, only using phantom values instead.

A phantom value is one which affects types, but which holds no information at runtime. As an example, let's say I wanted to define a units library - a classic example of phantom types. I could do that like this:

Quantity units data : [ Quantity units data ]

km : Num n -> Quantity [ Km ] (Num n)
km = \num ->
    Quantity Km num

cm : Num n -> Quantity [ Cm ] (Num n)
cm = \num ->
    Quantity Cm num

mm : Num n -> Quantity [ Mm ] (Num n)
mm = \num ->
    Quantity Mm num

add : Quantity u (Num n), Quantity u (Num n) -> Quantity u (Num n)
add = \Quantity units a, Quantity _ b ->
    Quantity units (a + b)

From a performance perspective, it's relevant here that [ Km ], [ Cm ], and [ Mm ] are all unions containing a single tag. That means they hold no information at runtime (they would always destructure to the same tag), which means they can be "unboxed" away - that is, discarded prior to code generation.

During code generation, Roc treats Quantity [ Km ] Int as equivalent to Quantity Int. Then, becaue Quantity Int is an alias for [ Quantity Int ], it will unbox again and reduce that all the way down to to Int.

This means that, just like phantom types, phantom values affect type checking only, and have no runtime overhead. Rust has a related concept called phantom data.

Modules and Shadowing

In Elm, my main module (where main lives) might begin like this:

module MyApp exposing (main)

import Parser
import Http exposing (Request)
import Task exposing (Task, after)

Roc application modules (where the equivalent of main lives) begin with the app keyword rather than the module keyword, and the import syntax is a bit different. Here's how the above module header would look in Roc:

app imports [ Parser, Http.{ Request }, Task.{ Task, after } ]

app modules are application entrypoints, and they don't formally expose anything. They also don't have names, so other modules can't even import them!

Modules that can be imported are interface modules. Their headers look like this:

interface Parser
    exposes [ Parser, map, oneOf, parse ]
    imports [ Utf8 ]

The name interface is intended to draw attention to the fact that the interface these expose is very important.

All imports and exports in Roc are enumerated explicitly; there is no .. syntax.

Since neither global tags nor private tags have a notion of "importing variants" (global tags are always available in all modules, and private tags are never available in other modules), there's also no exposing (Foo(..)) equivalent.

Like Elm, Roc does not allow shadowing.

Elm does permit overriding open imports - e.g. if you have import Foo exposing (bar), or import Foo exposing (..), you can still define bar = ... in the module. Roc considers this shadowing and does not allow it.

Record Syntax

Roc uses Rust/JavaScript syntax for record literals, e.g. { x: 1, y: 2 }.

It also allows omitting the value; { x, y } is sugar for { x: x, y: y }.

You can pattern match on exact record values, e.g. { x: 5 } ->.

Roc does not have the "a type alias for a record creates a convenience constructor function" feature that Elm has. This is partly because Point x y is already defined to be tag application in Roc, but also because { x, y } would be the recommended way to write it regardless.

Closed record annotations look the same as they do in Elm, e.g. { x : Int, y : Int }. Open record annotations look a bit different.

In Elm:

{ a | x : Int, y : Int } -> Int

In Roc:

{ x : Int, y : Int }* -> Int

Here, the open record's type variable appears attached to the }.

In the Elm example, the a is unbound, which in Roc means it appears as *.

Here's how that looks with a bound type variable. In Elm:

{ a | x : Int, y : Int } -> { a | x : Int, y : Int }

In Roc:

{ x : Int, y : Int }a -> { x : Int, y : Int }a

By design, this syntax makes the unbound case look natural and the bound case look unnatural.

That's because writing a function that accepts an open record with an unbound type variable (e.g. "this record, plus other fields if you like") is a totally reasonable thing to do - as often as you like! It has multiple upsides: it makes "named arguments" work with data model records more often, and makes it easier to change functions in backwards-compatible ways. It has no major downsides.

The syntax encourages doing this. "Just add a star" like so:

{ x : Int, y : Int }* -> Int

In contrast, using records with bound variables should be extremely rare.

They need to exist for the type system to work, and they aren't useless, but if you find yourself reaching for them, there is an extremely high chance that there's a better way to write that code.

The unnatural-looking syntax is the language's way of nudging you to reconsider, to search a little further for a better way to express things.

Record Update

Elm has "record update" syntax like this:

{ user | firstName = "Sam", lastName = "Sample" }

Roc has the same feature, but its syntax looks like this:

{ user & firstName: "Sam", lastName: "Sample" }

The record before the & can be qualified, like so:

{ Foo.defaultConfig & timeZone: utc }

However, it cannot involve record field access. So this would not compile:

{ Foo.defaults.config & timeZone: utc }

Optional Record Fields

There's a pattern in Elm where you pass a function a record of configuration values, some of which you don't really care about and want to leave as defaults. To incorporate the default config options, you call the function like so:

table { defaultConfig | height = 800, width = 600 }

This way, as the caller I'm specifying only the height and width fields, and leaving the others to whatever is inside defaultConfig. Perhaps it also has the fields x and y.

In Roc, you can do this like so:

table { height: 800, width: 600 }

...and the table function will fill in its default values for x and y. There is no need to use a defaultConfig record.

Here's how table would be defined in Roc:

table = \{ height, width, x ? 0.0, y ? 0.0 } ->

This is using optional field destructuring to destructure a record while also providing default values for any fields that might be missing. Here's the type of table:

table : { height : Float, width : Float, x ? Float, y ? Float } -> Table
table = \{ height, width, x ? 0.0, y ? 0.0 } ->

This says that table takes a record with two required fields (height and width and two optional fields (x and y). It also says that all of those fields have the type Float This means you can choose to omit x, y, or both, when calling the function...but if you provide them, they must be numbers.

This is also the type that would have been inferred for table if no annotation had been written. Roc's compiler can tell from the destructuring syntax x ? 0.0 that x is an optional field, and that it has the type Float. These default values can reference other expressions in the record destructure; if you wanted, you could write { height, width, x ? 0.0, y ? x + 1 }.

Destructuring is the only way to implement a record with optional fields. (For example, if you write the expression config.x and x is an optional field, you'll get a compile error.)

This means it's never possible to end up with an "optional value" that exists outside a record field. Optionality is a concept that exists only in record fields, and it's intended for the use case of config records like this. The ergonomics of destructuring mean this wouldn't be a good fit for data modeling.

Function equality

In Elm, if you write (\val -> val) == (\val -> val), you currently get a runtime exception which links to the == docs, which explain why this is the current behavior and what the better version will look like.

OCaml also has the "runtime exception if you compare functions for structural equality" behavior, but unlike Elm, in OCaml this appears to be the long-term design.

In Roc, function equality is a compile error, tracked explicitly in the type system. Here's the type of Roc's equality function:

'val, 'val -> Bool

Whenever a named type variable in Roc has a ' at the beginning, that means it is a functionless type - a type which cannot involve functions. If there are any functions in that type, it's a type mismatch. This is true whether val itself is a function, or if it's a type that wraps a function, like { predicate: (Int -> Bool) } or List (Bool -> Bool).

So if you write (\a -> a) == (\a -> a) in Roc, you'll get a type mismatch. If you wrap both sides of that == in a record or list, you'll still get a type mismatch.

If a named type variable has a ' anywhere in a given type, then it must have a ' everywhere in that type. So it would be an error to have a type like x, 'x -> Bool because x has a ' in one place but not everywhere.

Standard Data Structures

Elm has List, Array, Set, and Dict in the standard library.

Roc has List, Set, and Map in the standard library.

Here are the differences:

• List in Roc uses the term "list" the way Python does: to mean an ordered sequence of elements. Roc's List is more like an array, in that all the elements are sequential in memory and can be accessed in constant time. It still uses the [ ] syntax for list literals. Also there is no :: operator because "cons" is not an efficient operation on an array like it is in a linked list.
• Map in Roc is like Dict in Elm, except it's backed by hashing rather than ordering. Roc silently computes hash values for any value that can be used with ==, so instead of a comparable constraint on Set elements and Map keys, in Roc they instead have the functionless constraint indicated with a '. So to add to a Set you use Set.add : Set 'elem, 'elem -> Set 'elem, and putting a value into a Map is Map.put : Map 'key val, 'key, val -> Map 'key val.
• Set in Roc is like Set in Elm: it's shorthand for a Map with keys but no value, and it has a slightly different API.

The main reason it's called Map instead of Dict is that it's annoying to have a conversation about Dict out loud, let alone to teach it in a workshop, because you have to be so careful to enunciate. Map is one letter shorter, doesn't have this problem, is widely used, and never seems to be confused with the map function in practice (in e.g. JavaScript and Rust, both of which have both Map and map) even though it seems like it would in theory.

Roc also has a literal syntax for maps and sets. Here's how to write a Map literal:

{: "Sam" => 1, "Ali" => 2, firstName => 3 :}

This expression has the type Map Str Int, and the firstName variable would necessarily be a Str as well.

The Map literal syntax is for two reasons. First, Roc doesn't have tuples; without tuples, initializing the above Map would involve an API that looked something like one of these:

Map.fromList [ { k: "Sam", v: 1 }, { k: "Ali", v: 2 }, { k: firstName, v: 3 } ]

Map.fromList [ KV "Sam" 1, KV "Ali" 2, KV firstName 3 ]

This works, but is not nearly as nice to read.

Additionally, map literals can compile direcly to efficient initialization code without needing to (hopefully be able to) optimize away the intermediate List involved in fromList.

{::} is an empty Map.

You can write a Set literal like this:

[: "Sam", "Ali", firstName :]

The Set literal syntax is partly for the initialization benefit, and also for symmetry with the Map literal syntax.

[::] is an empty Set.

Roc does not have syntax for pattern matching on data structures - not even [ ] like Elm does.

Numbers

Like Elm, Roc has two numeric types: Int and Float. Float is the same as it is in Elm: it's a 64-bit floating point number. In Roc, Int is a 64-bit integer. Also unlike Elm, if you encounter an overflow with either of them, you get a runtime exception rather than wrapping overflow behavior (or a float becoming Infinity or -Infinity). You can opt into wrapping overflow instead with functions like Int.wrapAdd, or into a function that returns a Result that gives Err if it overflows, like Int.tryAdd.

Roc does not let floating point calculations result in Infinity, -Infinity, or NaN. Any operation which would result in one of these (such as sqrt or /) will result in a runtime exception. Similarly to overflow, you can opt into handling these a different way, such as Float.trySqrt which returns a Result.

The way + works here is also a bit different than in Elm. Imagine if Elm's (+) operator had this type:

Num a -> Num a -> Num a

Now imagine if Int were actually a type alias for Num Integer, and Float were actually a type alias for Num FloatingPoint. That's exactly how things work in Roc.

(Integer and FloatingPoint are both defined like Never; you can never instantiate one. They are used only as phantom types.)

So Roc does not use number, but rather uses Num - which works more like List. Either way, you get + being able to work on both Int and Float!

comparable, appendable, and number

These don't exist in Roc.

• appendable is only used in Elm for the (++) operator, and Roc doesn't have that operator.
• comparable is used for comparison operators (like < and such), plus List.sort, Dict, and Set. Roc's List.sort accepts a Sorter argument which specifies how to sort the elements. Roc's comparison operators (like <) only accept numbers; "foo" < "bar" is valid Elm, but will not compile in Roc. Roc's dictionaries and sets are hashmaps behind the scenes (rather than ordered trees), and their keys have no visible type restrictions.
• number is replaced by Num, as described earlier.

Like in Elm, number literals with decimal points are Float. However, number literals without a decimal point are Num * instead of number. Also like Python Roc permits underscores in number literals for readability purposes. Roc also supports hexadecimal (0x01), octal (0o01), and binary (0b01) integer literals; these literals all have type Int instead of Num *.

If you put these into a hypothetical Roc REPL, here's what you'd see:

> 1_024 + 1_024
2048 : Num *

> 1 + 2.14
3.14 : Float

> 1.0 + 1
2.0 : Float

> 1.1 + 0x11
<type mismatch between 1.1 : Float and 0x11 : Int>

> 11 + 0x11
28 : Int

Operators

In Elm, operators are functions. In Roc, all operators are syntax sugar.

This means, for example, that you cannot write (/) in Roc; that would be a syntax error. However, the / operator in Roc is infix syntax sugar for Num.div, which is a normal function you can pass to anything you like.

Elm has one unary operator, namely -. (In Elm, -x means "apply unary negate to x.") Roc has that one, and also unary !. The expression !foo desugars to not foo, and !foo bar desugars to not (foo bar).

This was introduced because Roc does not expose any functions globally by default (the way Elm does with Basics functions like not, round, etc.). In Roc, only operators and standard types (like Int and Bool) are exposed globally. Having to fully qualify not was annoying, and making an exception just for not seemed less appealing than making an operator for it, especially when unary ! is so widely used in other languages.

Because Roc has unary !, its "not equal to" operator is != instead of Elm's /=, for symmetry with unary !.

There's an Operator Desugaring Table at the end of this guide, so you can see exactly what each Roc operator desugars to.

The <| operator

Roc has no <| operator. (It does have |> though.)

In Elm, <| is used as a minor convenience for when you want to avoid some parens in a single-line expression (e.g. foo <| bar baz over foo (bar baz)) and as a major convenience when you want to pass an anonymous function, if, or case as an argument.

For example, elm-test relies on it:

test "it works" <|
    \_ -> ...

In Roc, this does not require a <|. This Roc code does the same thing as the preceding Elm code:

test "it works"
    \_ -> ...

You don't need parens or an operator to pass an anonymous function, when, or if as arguments. Here's another example:

foo 1 2 if something then 3 else 4

# Same as `foo 1 2 (if something then 3 else 4)`

CoffeeScript also does this the way Roc does.

Currying and |>

Roc functions aren't curried. Calling (List.append foo) is a type mismatch because List.append takes 2 arguments, not 1.

For this reason, function type annotations separate arguments with , instead of ->. In Roc, the type of List.take is:

List.take : List a, Int -> List a

You might also notice that Roc's List.take takes its arguments in the reverse order from how they are in Elm; the List is the first argument in Roc, whereas it would be the last argument in Elm. This is because Roc's |> operator works like Elixir's rather than like Elm's; here is an example of what it does in Roc:

a b c
    |> f x y

# f (a b c) x y

In Roc, the |> operator inserts the previous expression as the first argument to the subsequent expression, rather than as the last argument as it does in Elm.

This makes a number of operations more useful in pipelines. For example, in Roc, |> Num.div 2.0 divides by 2:

2000
  |> Num.div 2.0

# 1000.0 : Float

In Elm, where |> inserts 2 as the last argument, 2 ends up being the numerator rather than the denominator:

2000
  |> (/) 2.0

# 0.001 : Float

Another example is List.append. In Roc:

[ 1, 2 ]
  |> List.append [ 3, 4 ]

# [ 1, 2, 3, 4 ]

In Elm:

[ 1, 2 ]
  |> List.append [ 3, 4 ]

# [ 3, 4, 1, 2 ]

There are various trade-offs here, of course. Elm's |> has a very elegant implementation, and (|>) in Elm can be usefully passed to other functions (e.g. fold) whereas in Roc it's not even possible to express the type of |>.

As a consequence of |> working differently, "pipe-friendly" argument ordering is also different. That's why List.take has a "flipped" signature in Roc; otherwise, |> List.take 5 wouldn't work. Here's the type of Roc's List.take again, and also a pipeline using it:

List.take : List a, Int -> List a

[ 1, 2, 3, 4, 5 ]
    |> List.take 3

# The above expression gives the same answer it would in Elm.

Roc has no << or >> operators, and there are no functions in the standard library for general-purpose pointfree function composition.

Standard library

elm/core has these modules:

• Array
• Basics
• Bitwise
• Char
• Debug
• Dict
• List
• Maybe
• Platform
• Platform.Cmd
• Platform.Sub
• Process
• Result
• Set
• String
• Task
• Tuple

In Roc, the standard library is not a standalone package. It is baked into the compiler, and you can't upgrade it independently of a compiler release; whatever version of Roc you're using, that's the version of the standard library you're using too. (This is because Roc doesn't have a concept like Elm's Kernel; it would not be possible to ship Roc's standard library as a separate package!)

Roc's standard library has these modules:

• Bool
• Num
• Int
• Float
• List
• Map
• Set
• Bytes
• Str
• Result

Some differences to note:

• All these standard modules are imported by default into every module. They also expose all their types (e.g. Bool, Int, Result) but they do not expose any values - not even negate or not. (True, False, Ok, and Err are all global tags, so they do not need to be exposed; they are globally available regardless!)
• In Roc it's called Str instead of String.
• No Char. This is by design. What most people think of as a "character" is a rendered glyph. However, rendered glyphs are comprised of grapheme clusters, which are a variable number of Unicode code points - and there's no upper bound on how many code points there can be in a single cluster. In a world of emoji, I think this makes Char error-prone and it's better to have String be the smallest indivisible unit. If you want to iterate over grapheme clusters, use a Str -> List Str function which breaks the string down on grapheme boundaries. For this reason there also isn't a Str.length function; in the context of strings, "length" is ambiguous. (Does it refer to number of bytes? Number of Unicode code points? Number of graphemes?)
• No Basics. You use everything from the standard library fully-qualified; e.g. Bool.isEq or Num.add or Float.ceiling. There is no Never because [] already serves that purpose. (Roc's standard library doesn't include an equivalent of Basics.never, but it's one line of code and anyone can implmement it: never = \a -> never a.)
• No Tuple. Roc doesn't have tuple syntax. As a convention, Tup can be used to represent tuples (e.g. List.zip : List a, List b -> List [ Tup a b ]*), but this comes up infrequently compared to languages that have dedicated syntax for it.
• No Task. By design, platform authors implement Task (or don't; it's up to them) - it's not something that really could be usefully present in Roc's standard library.
• No Process, Platform, Cmd, or Sub - similarly to Task, these are things platform authors would include, or not.
• No Maybe. This is by design. If a function returns a potential error, use Result with an error type that uses a zero-arg tag to describe what went wrong. (For example, List.first : List a -> Result a [ ListWasEmpty ]* instead of List.first : List a -> Maybe a.) If you want to have a record field be optional, use an Optional Record Field directly (see earlier). If you want to describe something that's neither an operation that can fail nor an optional field, use a more descriptive tag - e.g. for a nullable JSON decoder, instead of nullable : Decoder a -> Decoder (Maybe a), make a self-documenting API like nullable : Decoder a -> Decoder [ Null, NonNull a ]*.
• List refers to something more like Elm's Array, as noted earlier.

Operator Desugaring Table

Here are various Roc expressions involving operators, and what they desugar to.

Expression	Desugars to
a + b	Num.add a b
a - b	Num.sub a b
a * b	Num.mul a b
a / b	Num.div a b
a // b	Num.divFloor a b
a ^ b	Num.pow a b
a % b	Num.rem a b
a %% b	Num.mod a b
-a	Num.neg a
-f x y	Num.neg (f x y)
a == b	Bool.isEq a b
a != b	Bool.isNotEq a b
a && b	Bool.and a b
a || b	Bool.or a b
!a	Bool.not a
!f x y	Bool.not (f x y)
a |> b	b a
a b c |> f x y	f (a b c) x y