mirror of
https://github.com/enso-org/enso.git
synced 2024-12-19 04:01:46 +03:00
148 lines
5.3 KiB
Markdown
148 lines
5.3 KiB
Markdown
---
|
|
layout: developer-doc
|
|
title: Function Arguments
|
|
category: syntax
|
|
tags: [syntax, functions]
|
|
order: 11
|
|
---
|
|
|
|
# Function Arguments
|
|
|
|
One of the biggest usability innovations of Enso is the set of argument types
|
|
that it supports. The combination of named and defaulted arguments with a
|
|
curried language creates a tool in which it is very clear to express even
|
|
complex APIs.
|
|
|
|
<!-- MarkdownTOC levels="2,3" autolink="true" -->
|
|
|
|
- [Positional Arguments](#positional-arguments)
|
|
- [Named Arguments](#named-arguments)
|
|
- [Defaulted Arguments](#defaulted-arguments)
|
|
- [Optional Arguments](#optional-arguments)
|
|
- [Splats Arguments \(Variadics\)](#splats-arguments-variadics)
|
|
- [Type Applications](#type-applications)
|
|
- [Underscore Arguments](#underscore-arguments)
|
|
|
|
<!-- /MarkdownTOC -->
|
|
|
|
## Positional Arguments
|
|
|
|
Much like most programming languages, functions in Enso can be called with their
|
|
arguments provided positionally. This is the simple case that everybody is
|
|
familiar with.
|
|
|
|
## Named Arguments
|
|
|
|
All arguments in Enso are defined with a name. Like all programming languages,
|
|
this is necessary for that argument to be used. However, what Enso allows is for
|
|
users to then _call_ those arguments by name.
|
|
|
|
- An argument is called by name using the syntax `(name = value)` (or one may
|
|
also take advantage of the operator precedence to write `name=value`).
|
|
- Named arguments are applied in the order they are given. This means that if
|
|
you positionally apply to an argument `foo` and then try to later apply to it
|
|
by name, this will fail due to currying of functions.
|
|
- Named arguments _cannot_ be used while using operator syntax. This means that
|
|
an expression of the form `a + b` cannot apply arguments by name. However,
|
|
when calling the operator as a method (`a.+ b`), the call-by-name syntax may
|
|
indeed be used (`a.+ (that = b)`).
|
|
|
|
This is a great usability boon as in complex APIs it can often be difficult to
|
|
remember the order or arguments.
|
|
|
|
## Defaulted Arguments
|
|
|
|
Enso also allows users to define their functions with _defaults_ for the
|
|
function's arguments. This is very useful for complex APIs as it allows users to
|
|
experiment and iterate quickly by only providing the arguments that they want to
|
|
customise.
|
|
|
|
- An argument is defined with a default using the syntax `(name = default_val)`,
|
|
which, as above, accounts for precedence rules.
|
|
- Argument defaults are applied to the function if no argument value is provided
|
|
by position or name for that argument.
|
|
- Argument defaults are evaluated lazily if the function is lazy in that
|
|
argument.
|
|
- We provide a `...` operator which suspends application of the default
|
|
arguments for the purposes of currying.
|
|
|
|
## Optional Arguments
|
|
|
|
There are certain cases where the type information for an argument may be able
|
|
to be inferred by the compiler. This is best explained by example. Consider the
|
|
implementation of a `read` function that reads text and outputs a value of a
|
|
particular type.
|
|
|
|
```ruby
|
|
read : Text -> t -> t
|
|
read text this = t.fromText text
|
|
```
|
|
|
|
You can use this function by explicitly providing the type information in either
|
|
of the following ways:
|
|
|
|
```ruby
|
|
val1 = read '5' Int
|
|
val2 = Int.read '5'
|
|
```
|
|
|
|
This, however, is often tedious, especially in contexts where this information
|
|
could be inferred by the compiler. We can re-write `read` as follows:
|
|
|
|
```ruby
|
|
read : Text -> (t=t) -> t
|
|
read text (this=this) = t.fromText text
|
|
```
|
|
|
|
This allows users both to provide the argument explicitly or leave it out. In
|
|
the case where it is not provided, the compiler will attempt to infer it from
|
|
usage. If this is impossible, an error would be raised.
|
|
|
|
Enso provides a syntactic sugar for the `t=t` syntax. The above code can be
|
|
written instead using `?`.
|
|
|
|
```ruby
|
|
read : Text -> t? -> t
|
|
read text this? = t.fromText text
|
|
```
|
|
|
|
## Splats Arguments (Variadics)
|
|
|
|
Enso provides users with the ability to define variadic functions, or _splats_
|
|
functions in our terminology. These are very useful for defining expressive APIs
|
|
and flexible code.
|
|
|
|
- These work for both positional and keyword arguments.
|
|
- They are defined using the syntax `name...`, where `name` is an arbitrary
|
|
argument name.
|
|
|
|
> The actionables for this section are:
|
|
>
|
|
> - Work out how (and if) this can interact with currying.
|
|
> - Do we even want this?
|
|
|
|
## Type Applications
|
|
|
|
There are sometimes cases where the user wants to explicitly refine the type of
|
|
an argument at the _call_ site of a function. This can be useful for debugging,
|
|
and for writing ad-hoc code. Much like the named-arguments in applications
|
|
above, Enso also provides a syntax for refining types at the application site.
|
|
|
|
- To refine an argument type by name at the application site, use the `:=`
|
|
operator (e.g. `arg_name := T`).
|
|
- This _will_ be type-checked by the compiler, and so `T` must be a valid
|
|
subtype for the type inferred for (or defined for) the function being called.
|
|
|
|
## Underscore Arguments
|
|
|
|
Enso provides the `_` argument as a quick way to create a lambda from a function
|
|
call. It obeys the following rules.
|
|
|
|
- Replacing any function argument with `_` will create a lambda that accepts an
|
|
argument and passes it in the place of the underscore. All other function
|
|
arguments are applied as normal.
|
|
- This works both by name and positionally.
|
|
- When a function is provided multiple `_` arguments, they are desugared left to
|
|
right as the arguments would be applied to the function definition, creating
|
|
nested lambdas.
|