47 KiB
Roc for Elm programmers
Roc is a direct descendant of the Elm programming language. The two languages are similar, but not the same!
This is a guide to help Elm programmers learn what's different between Elm and Roc.
NOTE: Almost all that's in this document has been implemented - but not quite all of it!
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.
Casing
Like Elm, Roc uses camelCase for identifiers. Modules and types begin with a capital letter (eg.
Decode), variables (including type variables!) begin with a lower case letter (eg. request).
Acronyms also use camelCase despite being capitalized in English, eg. xmlHttpRequest
for a
variable and XmlHttpRequest
for a type. Each word starts with a capital letter, so if acronyms are
only capitals it's harder to see where the words start. eg. XMLHTTPRequest
is less clear than
XmlHttpRequest
, unless you already know the acronyms.
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 don't come up.
Roc strings also have the type Str
rather than Elm's String
. This is to make
common qualified operations like Str.join
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, consider this Elm code:
numbers =
let
num1 =
123
num2 =
456
in
[num1, num2]
Here's the equivalent Roc code:
numbers =
num1 =
123
num2 =
456
[num1, num2]
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.
Roc has a built-in formatter that has a lot in common with elm-format
(e.g. no configuration,
no enforced line length) but also some stylistic differences. One notable difference is that
it doesn't use as much spacing. For example, if you ran roc format
on the following Roc
code, the formatter would not change it:
numbers =
num1 = 123
num2 = 456
[num1, num2]
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 parentheses to destructure arguments inline. For example, in Elm, you always need to use parens 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 write the above like so in Roc:
\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 [] : List a
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)
. The equivalent Roc type would be:
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".
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. However, it does allow trailing commas, in both values and
type annotations. Roc's formatter (which is built into the compiler, and which
is zero-configuration like elm-format
) formats multi-line record literals (and
record types) with a comma at the end of each line, like so:
user = {
firstName: "Sam",
lastName: "Sample",
email: "sam@example.com",
}
This is easy to read and leads to tidy version control diffs; no matter how you reorder these fields, or add or remove fields, the diffs will only be of the relevant fields in question, not any adjacent fields or tokens.
Lists are formatted similarly to records, except of course they don't have labeled fields.
Closed record annotations look the same as they do in Elm, e.g.
{ name : Str, email : Str }
. Open record annotations look a bit different.
In Elm:
{ a | name : Str, email : Str } -> Str
In Roc:
{ name : Str, email : Str }* -> Str
Here, the open record's type variable appears immediately after the }
.
In the Elm example, the
a
is unbound, which in Roc means it appears as*
.
This syntax makes it easier to write a function that accepts an open record with an unbound type variable (e.g. "this record, plus other fields if you like"). This 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:
{ name : Str, email : Str }* -> Str
You can also use bound type variables too. In Elm:
{ a | name : Str, email : Str } -> { a | name : Str, email : Str }
In Roc:
{ name : Str, email : Str }a -> { name : Str, email : Str }a
Like in Elm, 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 any time you find yourself reaching for them, there is a very high chance that there's a better way to write that code!
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 title
and description
.
In Roc, you can do this like so:
table { height: 800, width: 600 }
...and the table
function will fill in its default values for title
and description
.
There is no need to use a defaultConfig
record.
Here's how that table
function would be implemented in Roc:
table = \{ height, width, title ? "", description ? "" } ->
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 : Pixels,
width : Pixels,
title ? Str,
description ? Str,
}
-> Table
table = \{ height, width, title ? "", description ? "" } ->
This says that table
takes a record with two required fields (height
and
width
and two optional fields (title
and description
). It also says that
the height
and width
fields have the type Pixels
(a type alias for some
numeric type), whereas the title
and description
fields have the type Str
.
This means you can choose to omit title
, description
, or both, when calling
the function...but if you provide them, they must have the type Str
.
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
title ? ""
that title
is an optional field, and that it has the type Str
.
These default values can reference other expressions in the record destructure; if you
wanted, you could write
{ height, width, title ? "", description ? Str.concat "A table called " title }
.
Destructuring is the only way to implement a record with optional fields.
(For example, if you write the expression config.title
and title
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.
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 ofcase
...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
Like Rust, you can add an if
guard after a pattern:
when color is
Blue -> 1
Green | Red | Yellow if totalColors >= 3 -> 2
Green | Red | Yellow -> 4 # only gets run if totalColors < 3
This gives you a way to use constants in patterns:
pi = 3.14
e = 2.72
when number is
0 -> "zero"
1 -> "one"
v if v == pi -> "pi"
v if v == e -> "e"
_ -> ""
Booleans
Roc has a Bool
module (with operations like Bool.and
and Bool.or
; Roc does not have
a Basics
module), and Bool
is an opaque type. The values Bool.true
and Bool.false
work
like True
and False
do in Elm.
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 an 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,
and that Task.await
is like Elm's Task.andThen
but with the arguments
flipped, here's one way I might write this:
doStuff = \filename ->
Task.await (File.read filename) \fileData ->
Task.await (Http.get (urlFromData fileData)) \response ->
File.write filename (responseToData response)
Note that in Elm you'd need to add a
<|
before the anonymous functions (e.g.<| \response ->
) but in Roc you don't.
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
Task.await : 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 await
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 -> ...
ReadAccessDenied -> ...
FileCorrupted -> ...
# File.WriteErr possibilities
DirectoryNotFound -> ...
WriteAccessDenied -> ...
DiskFull -> ...
Here is the set of slightly different types that will make the original chained
expression compile. (await
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 *)
await : 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 those look:
Http.Err a : [
PageNotFound,
Timeout,
BadPayload Str,
]a
File.ReadErr a : [
FileNotFound,
Corrupted,
BadFormat,
]a
File.WriteErr a : [
FileNotFound,
DiskFull,
]a
For a side-by-side comparison, here's how we would implement something similar in Elm:
type Http.Err
= PageNotFound
| Timeout
| BadPayload String
type File.ReadErr
= FileNotFound
| Corrupted
| BadFormat
type File.WriteErr
= FileNotFound
| DiskFull
There are a few differences between them, but the most significant one here is that the Roc version has a type variable.
That type variable has a similar purpose to the type variable in Elm's open records
(e.g. the a
in { a | name : String, email : String }
which in Roc would be
{ name : Str, email : Str }a
) - except applied to sum types (such as
Elm's custom types) instead of product types (such as records).
If you were to remove the type variables from the Roc declarations for
Http.Err
,File.ReadErr
, andFile.WriteErr
, they would work practically the same way as the Elm one. Roc tag unions can be used as traditional algebraic data types, and they have the usual support for pattern matching, exhaustiveness checking, and so on.
You don't need to declare tag unions before using them. Instead, you can just write a tag (essentially a variant) anywhere you like, and Roc will infer the type of the union it goes in.
Here are some examples of using tags in a REPL:
> Red
Red : [Red]*
> Blue
Blue : [Blue]*
> Ok "hi"
Ok "hi" : [Ok Str]*
> SomethingIJustMadeUp "hi" "there"
SomethingIJustMadeUp "hi" "there" : [SomethingIJustMadeUp Str Str]*
> x = Foo
Foo : [Foo]*
> y = Foo "hi" Bar
Foo "hi" Bar : [Foo Str [Bar]*]*
> z = Foo ["str1", "str2"]
Foo ["str1", "str2"] : [Foo (List Str)]*
The [
]
s in the types are tag unions, and they list all the possible
different tags that the value could be at runtime. In all of these tag unions,
there is only one tag. Notice the *
at the end; that's the type variable
we saw earlier.
Similarly to how if you put { name = "" }
into elm repl
, it will
infer a type of { a | name : String }
- that is, an open record with an
unbound type variable and name : Str
field - if you put a tag Foo ""
into
roc repl
, it will infer a type of [Foo Str]*
- that is, an open tag union
with one alternative: a Foo
tag with a Str
payload.
The same tag can be used with different arities and types. In the REPL above,
x
, y
, and z
, can all coexist even though they use Foo
with different
arities - and also with different types within the same arity.
Similarly, you can pattern match on tags without declaring any types, and Roc will infer the type of the tag union being matched.
For example, suppose we don't write any type annotations anywhere, and have this pattern match:
when blah is
MyStr str -> Str.concat str "!"
MyBool bool -> Bool.not bool
The inferred type of this expression would be [MyStr Str, MyBool Bool]
.
Exhaustiveness checking is still in full effect here. It's based on usage; if any code pathways led to
blah
being set to the tagFoo
, I'd get an exhaustiveness error because thiswhen
does not have aFoo
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 Str]*
y = Bar "stuff"
tagToStr : [Foo, Bar Str] -> Str
tagToStr = \tag ->
when tag is
Foo -> "hi"
Bar str -> Str.concat str "!"
Each of these type annotations involves a tag union - a collection of tags bracketed by [
and ]
.
- The type
[Foo, Bar Str]
is a closed tag union. - The type
[Foo]*
is an open tag union.
You can pass x
to tagToStr
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 tagToStr
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 Str]* -> [Foo Str]*
alwaysFoo = \tag ->
when tag is
Foo str -> Foo (Str.concat str "!")
_ -> Foo ""
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 Str]*
based on usage.
Just because [Foo Str]*
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 Str, Bar Bool] -> [Foo Str]*
...then the function would only accept tags like Foo "hi"
and Bar (x == y)
. 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. 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:
exclaimFoo : [Foo Str]a -> [Foo Str]a
exclaimFoo = \tag ->
when tag is
Foo str -> Foo (Str.concat str "!")
other -> other
The *
says "this union can also include any other tags", and here the a
says
"the return value union includes Foo Str
, plus whichever other tags the argument
includes in its union."
The Roc type
[]
is equivalent to Elm'sNever
. You can never satisfy it!
Opaque Types
In Elm, you can choose to expose (or not) custom types' constructors in order to create opaque types. Since Roc's tags can be constructed in any module without importing anything, Roc has a separate opaque type language feature to enable information hiding.
As an example, suppose I define these inside the Username
module:
Username := Str
fromStr : Str -> Username
fromStr = \str ->
@Username str
toStr : Username -> Str
toStr = \@Username str ->
str
Here, Username
is an opaque type. The fromStr
function turns a string into a Username
by "calling" @Username
on that string. The toStr
function turns a Username
back into
a string by pattern matching @Username str ->
to unwrap the string from the Username
.
Now that I've defined the Username
opaque type, I can expose it so that other modules can use
it in type annotations. However, other modules can't use the @Username
syntax to wrap or unwrap
Username
values. That operation is only available in the same scope where Username
itself was
defined; trying to use it outside that scope will give an error.
If I were to define
Username := Str
inside another module (e.g.Main
) and use@Username
, it would compile, but thatUsername
opaque type would not be considered equal to the one defined in theUsername
module. Although both opaque types have the nameUsername
, they were defined in different modules. That means the twoUsername
types would be type-incompatible with each other, and even attempting to use==
to compare them would be a type mismatch.
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, await)
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 imports section would look in Roc:
app imports [Parser, Http.{ Request }, Task.{ Task, await }]
app
modules are application entry points, 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 tags are available in all modules, Roc does not have a notion of "importing variants", and there's also no
exposing (Foo(..))
equivalent. (Later on, we'll talk about how opaque types work in Roc.)
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 treats this as shadowing and does not allow it.
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 too, and also unary !
.
The expression !foo
desugars to Bool.not foo
, and !foo bar
desugars
to Bool.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 Str
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.
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.map
is:
List.map : List a, (a -> b) -> List b
You might notice that Roc's List.map
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
In Elm, where |>
inserts 2 as the last argument, 2 ends up being the numerator
rather than the denominator:
2000
|> (/) 2.0
# 0.001
Another example is List.append
, which is called List.concat
in Roc:
[1, 2]
|> List.concat [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.map
has a "flipped" signature in Roc; otherwise, |> List.map Num.abs
wouldn't work on a list of numbers. Here's the type of Roc's List.map
again, and also a pipeline using it:
List.map : List a, (a -> b) -> List b
[-1, 2, 3, -4]
|> List.map Num.abs
Roc has no <<
or >>
operators, and there are no functions in the standard library
for general-purpose pointfree function composition.
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" <|
\_ -> verify stuff
In Roc, this does not require a <|
. This Roc code does the same thing as the preceding Elm code:
test "it works"
\_ -> verify stuff
This is convenient with higher-order functions which take a function as their final argument. Since many Roc functions have the same type as Elm functions except with their arguments flipped, this means it's possible to end a lot of expressions with anonymous functions - e.g.
modifiedNums =
List.map nums \num ->
doubled = num * 2
modified = modify doubled
modified / 2
Separately, you don't need parens or an operator to pass 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.
Backpassing
Suppose I'm using a platform for making a CLI, and I want to run several
Task
s in a row which read some files from the disk. Here's one way I could do
that, assuming Task.await
is like Elm's Task.andThen
with arguments flipped:
readLicense : Filename -> Task License File.ReadErr
readLicense = \filename ->
Task.await (File.readUtf8 settingsFilename) \settingsYaml ->
when Yaml.decode settingsDecoder settingsYaml is
Ok settings ->
Task.await (File.readUtf8 settings.projectFilename) \csv ->
when Csv.decode projectDecoder csv is
Ok project ->
Task.await (File.readUf8 project.licenseFilename) \licenseStr ->
when License.fromStr licenseStr is
Ok license -> Task.succeed license
Err err -> Task.fail (InvalidFormat err)
Err err -> Task.fail (InvalidFormat err)
Err err ->
Task.fail (InvalidFormat err)
We can write this with |>
instead of when
like so:
readLicense : Filename -> Task License File.ReadErr
readLicense = \filename ->
Task.await (File.readUtf8 settingsFilename) \settingsYaml ->
settingsYaml
|> Yaml.decode settingsDecoder
|> Task.fromResult
|> Task.mapFail InvalidFormat
|> Task.await \settings ->
Task.await (File.readUtf8 settings.projectFilename) \projectCsv ->
projectCsv
|> Csv.decode projectDecoder
|> Task.fromResult
|> Task.mapFail InvalidFormat
|> Task.await \project ->
Task.await (File.readUf8 project.licenseFilename) \licenseStr ->
License.fromStr licenseStr
|> Task.fromResult
|> Task.mapFail InvalidFormat
We can also write it this way, which is equivalent to the previous two ways:
readLicense : Filename -> Task License File.ReadErr
readLicense = \filename ->
settingsYaml <- Task.await (File.readUtf8 settingsFilename)
settings <-
settingsYaml
|> Yaml.decode settingsDecoder
|> Task.fromResult
|> Task.mapFail InvalidFormat
projectCsv <- Task.await (File.readUtf8 settings.projectFilename)
project <-
projectCsv
|> Csv.decode projectDecoder
|> Task.fromResult
|> Task.mapFail InvalidFormat
licenseStr <-
Task.await (File.readUf8 project.licenseFilename)
License.fromStr licenseStr
|> Task.fromResult
|> Task.mapFail InvalidFormat
This uses backpassing syntax to nest anonymous functions without indenting them. Here's a smaller demonstration of backpassing; the second snippet is sugar for the first.
list =
List.map numbers \num -> num + 1
list =
num <- List.map numbers
num + 1
Both snippets are calling List.map
passing numbers
as the first argument,
and a \num -> num + 1
function for the other argument.
The difference is that in the second snippet, the \num -> num + 1
function is
written backwards, like this:
num <-
num + 1
This is called backpassing because you write the function backwards and then immediately pass it as an argument to another function.
The other function - the one you're passing this one to - goes right after
the <-
symbol. That function should be called with one argument missing at
the end, such as with List.map numbers
(which is missing its final argument).
Here's another pair of snippets, this time using two backpassing calls:
incrementedNumbers =
List.map lists \numbers ->
List.map numbers \num -> num + 1
incrementedNumbers =
numbers <- List.map lists
num <- List.map numbers
num + 1
In the second snippet, we have two functions defined in the backpassing style. The first function is:
numbers <-
num <- List.map numbers
num + 1
This function desugars to \numbers -> …
and is being passed as the final argument
to List.map lists
.
The second function defined in backpassing style is:
num <-
num + 1
This function desugars to \numbers -> …
and is being passed as the final argument
to List.map numbers
. That List.map numbers
call is the body of
the numbers <-
function we defined in backpassing style a moment ago - so
in a normal function definition, it would be \numbers -> List.map numbers …
Note that backpassing can be combined with the |>
operator, which lets you
call a function with two arguments missing from the end - one provided by
the |>
and the other provided by the <-
, like so:
incrementedNumbers =
num <-
[1, 2, 3]
|> List.reverse
|> List.map
num + 1
Here, the first argument to List.map
is provided by the |>
(namely the reversed [1, 2, 3]
list), and the second argument is provided by +the <-
(namely the \num -> …
function).
Backpassing can also be used with functions that take multiple arguments; for
example, you could write key, value <- Dict.map dictionary
similarly to how
we used List.map
here. That would desugar into a
Dict.map dictionary \key, value -> …
function.
To be clear, backpassing is designed to be used with chaining functions like
Task.await
which are prone to lots of nesting. It isn't designed to be used with functions likeList.map
; this is just a simplified example to show that<-
can be used with any function...even those where it doesn't improve code clarity!
Finally, here's an example combining backpassing with ordinary =
definitions:
task =
user <- Task.await fetchUser
url = user.baseUrl
settings, bio, posts <- Task.map3 (getSettings url) (getBio url) (getPosts url)
profile = makeProfile settings bio
Task.succeed { profile, posts }
Here, every new name that's introduced to scope is aligned on the left-hand edge
of the expression - regardless of whether it's coming from =
or from <-
.
Numbers
Like Elm, Roc organizes numbers into integers and floating-point numbers. However, Roc breaks them down even further. For example, Roc has two different sizes of float types to choose from:
F64
- a 64-bit IEEE 754 binary floating point numberF32
- a 32-bit IEEE 754 binary floating point number
Both types are desirable in different situations. For example, when doing
simulations, the precision of the F64
type is desirable. On the other hand,
GPUs tend to heavily prefer 32-bit floats because a serious bottleneck is how
long it takes data to transfer from CPU to GPU, so having to send half as many
bytes per render (compared to 64-bit floats) can be huge for performance.
Roc also supports Dec
, which is a 128-bit fixed-point decimal number. Dec
is decimal-based, so 0.1 + 0.2 == 0.3
(whereas in binary floats this is not true),
which makes it much better for calculations involving currency, among other use cases.
The downside of Dec
is that it does not have hardware support, so calculations involving
them take longer than they do with floats.
Similarly to how there are different sizes of floating point numbers, there are also different sizes of integer to choose from:
I8
I16
I32
I64
I128
Roc also has unsigned integers which are never negative. They are
U8
, U16
, U32
, U64
, U128
, and Nat
.
The size of Nat
depends on what target you're building for; on a 64-bit target
(the most common), at runtime Nat
will be the same as U64
, whereas on a 32-bit
target (for example, WebAssembly) at runtime it will be the same as U32
instead.
Nat
comes up most often with collection lengths and indexing into collections.
For example:
List.len : List * -> Nat
List.get : List elem, Nat -> Result elem [OutOfBounds]*
List.set : List elem, Nat, elem -> List elem
As with floats, which integer type to use depends on the values you want to support
as well as your performance needs. For example, raw sequences of bytes are typically
represented in Roc as List U8
. You could also represent them as List U128
,
but it's much more efficient to use List U8
, since each byte will be at most 255 anyway.
Like Elm, it's possible in Roc to have functions that work on either integers
or floating-point numbers. However, the types are different. For example,
the type of Num.add
(which the +
operator desugars to) is:
Num.add : Num a, Num a -> Num a
This accepts any of the numeric types discussed above, from I128
to F32
to D64
and everything in between. This is because those are all type aliases
for Num
types. For example:
I64
is a type alias forNum (Integer Signed64)
U8
is a type alias forNum (Integer Unsigned8)
F32
is a type alias forNum (Fraction Binary32)
Dec
is a type alias forNum (Fraction Decimal)
(Those types like Integer
, Fraction
, and Signed64
are all 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 integers and floats!
Separately, there's also Int a
, which is a type alias for Num (Integer a)
,
and Frac a
, which is a type alias for Num (Fraction a)
. These allow functions
that can work on any integer or any fractional number. For example,
Num.bitwiseAnd : Int a, Int a -> Int a
.
In Roc, number literals with decimal points are Frac *
values.
Number literals without a decimal point are Num *
values. Almost always these
will end up becoming something more specific, but in the unlikely event
(most often in a REPL) that you actually do end up with an operation that runs
on either an Int *
or a Num *
value, it will default to being treated as
an I64
. Similarly, a Frac *
value will default to being treated as a D64
,
which means if someone is learning Roc as their first programming language and
they type 0.1 + 0.2
into a REPL, they won't be confused by the answer.
If you encounter integer or Dec
overflow in Roc, by default you get a runtime
exception. You can opt into wrapping integer overflow instead with functions like
Num.addWrap : Int a, Int a -> Int a
, or use a function that gives Err
if it
overflows, like Num.addChecked : Num a, Num a -> Result (Num a) [Overflow]*
.
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 : Frac *
> 1.0 + 1
2.0 : Frac *
> 1.1 + 0x11
<type mismatch between `1.1 : Frac *` and `0x11 : Int *`>
> 11 + 0x11
28 : Int *
Testing
Instead of a separate testing tool, Roc has a built-in expect
keyword, which
you can use in conjunction with roc test
to run tests.
See the tutorial section on testing for details.
Abilities
comparable
, appendable
, and number
don't exist in Roc.
number
is replaced byNum
, as described previously.appendable
is only used in Elm for the(++)
operator, and Roc doesn't have that operator.comparable
is used in Elm for comparison operators (like<
and such), plusList.sort
,Dict
, andSet
. 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), so their keys need to be hashable but not necessarily comparable.
That said, Roc's Dict
and Set
do have a restriction on their keys, just not comparable
.
See the section on Abilities in the tutorial for details.
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:
Str
Bool
Num
List
Dict
Set
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
,List
,Result
) but they do not expose any values - not evennegate
ornot
. (Ok
andErr
are ordinary tags, so they do not need to be exposed; they are globally available regardless!) - In Roc it's called
Str
instead ofString
. List
refers to something more like Elm'sArray
, as noted earlier.- 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 makesChar
error-prone and it's better to haveStr
be the only first-class unit. For convenience when working with unicode code points (e.g. for performance-critical tasks like parsing), the single-quote syntax is sugar for the correspondingU32
code point - for example, writing'鹏'
is exactly the same as writing40527
. Like Rust, you get a compiler error if you put something in single quotes that's not a valid Unicode scalar value. - No
Basics
. You use everything from the standard library fully-qualified; e.g.Bool.not
orNum.negate
orNum.ceiling
. There is noNever
because[]
already serves that purpose. (Roc's standard library doesn't include an equivalent ofBasics.never
, but it's one line of code and anyone can implement it:never = \a -> never a
.) - No
Tuple
module, but there is syntax support for tuples which allows not only destructuring (like in Elm) but also direct field access - which looks like record field access, but with numbered indices instead of named fields. For example, the Elm codeTuple.first ( "a", "b" )
andTuple.second ( "a", "b" )
could be written in Roc as("a", "b").0
and("a", "b").1
. Roc tuples can also have more than two fields. - No
Task
. By design, platform authors implementTask
(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
, orSub
- similarly toTask
, these are things platform authors would include, or not. - No
Debug
. Roc has a built-indbg
keyword instead ofDebug.log
and acrash
keyword instead ofDebug.todo
. - No
Maybe
. This is by design. If a function returns a potential error, useResult
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 ofList.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 ofnullable : Decoder a -> Decoder (Maybe a)
, make a self-documenting API likenullable : Decoder a -> Decoder [Null, NonNull a]*
.
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.divTrunc a b |
a ^ b |
Num.pow a b |
a % b |
Num.rem a b |
a >> b |
Num.shr a b |
a << b |
Num.shl 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 |