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149 lines
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149 lines
5.0 KiB
Markdown
---
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layout: developer-doc
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title: Assignment Expressions
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category: syntax
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tags: [syntax, assignment]
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order: 6
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---
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# Assignment Expressions
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Assignment syntax in Enso is fairly magical, given that it is the language's
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syntax for monadic bind.
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<!-- MarkdownTOC levels="2,3" autolink="true" -->
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- [How Assignment Works](#how-assignment-works)
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- [Function Definitions](#function-definitions)
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- [Pattern Match Bindings](#pattern-match-bindings)
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- [Extension Methods](#extension-methods)
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- [Method Syntax](#method-syntax)
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- [Function Syntax](#function-syntax)
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- [Top-Level Assignments](#top-level-assignments)
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<!-- /MarkdownTOC -->
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## How Assignment Works
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Assignment in Enso operates as follows:
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- An assignment is an _expression_.
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- The left-hand-side introduces a pattern context.
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- The pattern on the left-hand-side is matched against (unified with) the value
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that occurs on its right-hand-side.
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- A single line must contain at most one assignment.
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- An assignment may only appear as the _root expression_ of a line of code in a
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file.
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- An assignment returns the value `Nothing`, and does not return the value that
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is assigned to it.
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The assignment operator has myriad uses, and is used to define variables,
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functions, extension methods, and to perform pattern matching. Each different
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case will see an appropriate desugaring applied (see below).
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Please note that not _all_ occurrences of the `=` operator are assignments in
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the general sense. The above rules do not apply when using said operator to pass
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arguments by name.
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## Function Definitions
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If the left hand side of an assignment is syntactically a prefix application
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chain, where the left-most name is a _variable_ name, the assignment is
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considered to introduce a _function definition_ (the syntax sugared version).
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For a prefix chain `a b c = ...`, this operates as follows:
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- The name `a` is bound in the enclosing scope, and is called the 'function
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name'.
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- The names `b` and `c` (the 'function arguments') are converted into nested
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lambda arguments in the function body.
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In essence, the above example is equivalent to:
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```ruby
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a = b -> c -> ...
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```
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Please note that by the rules of naming specified previously, if an operator
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occurs in the same position as `a` it will also be defined.
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## Pattern Match Bindings
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If the left hand side of an assignment is syntactically a prefix application
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chain, where the left-most name is a _type_ name, the assignment is considered
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to introduce a pattern match binding.
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It operates as follows for code consisting of a prefix chain `A b c = expr` and
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trailing code `tail...`.
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```ruby
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A b c = expr
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tail...
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```
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- A case expression is created with scrutinee `expr`.
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- The matching names `A`, `b`, and `c` are used in a case expression branch's
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pattern. The branch's expression is `tail...`.
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- A catch-all branch is created that has expression `error`.
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As each branch in a case expression has its own scope, this desugaring means
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that the names `b` and `c` are made visible in the scope where the pattern match
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binding occurs. This is due to the fact that pattern match branches are lambda
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expressions, and reuse the same scoping rules.
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This also works for operators in an infix position, where its operands will be
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matched against.
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## Extension Methods
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There are two cases where an assignment creates an extension method:
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1. **Method Syntax:** If the left-hand-side of an assignment is syntactically a
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prefix application chain where the left-most expression is an infix
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application of `.`, this assignment is considered to introduce an extension
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method.
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2. **Function Syntax:** If the left hand side of an assignment is syntactically
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a prefix application chain where the left-most expression is a variable
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identifier and the second expression from the left is a variable named `this`
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with an explicit type ascription, this is also considered to introduce an
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extension method.
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### Method Syntax
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This syntax for extension methods works as follows:
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- The target of the method syntax (left argument to `.`) defines the type on
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which the extension method is created.
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- An implicit `this` argument is inserted with that type at the start of the
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arguments list.
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- All arguments are desugared to lambda arguments.
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```ruby
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My_Type.method_name a b c = ...
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```
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### Function Syntax
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This syntax for extension methods works as follows:
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- The `this` argument type is used to define the type on which the extension
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method is created.
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- `this` and all remaining arguments are desugared to lambda arguments.
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```ruby
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method_name (this : My_Type) a b c = ...
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```
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## Top-Level Assignments
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In order to aid with disambiguation, any binding made in the root scope without
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an explicit target is implicitly defined on a type representing the current
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module. For example, a binding `main = ...` is implicitly `here.main = ...`.
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This works as follows:
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- All non-extension methods defined at the top level are augmented with an
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implicit first parameter `here`.
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- They are callable by `name` if not ambiguous, but can be disambiguated by
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using `here.name` where necessary.
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