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151 lines
4.2 KiB
Markdown
151 lines
4.2 KiB
Markdown
---
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layout: developer-doc
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title: Projections and Field Access
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category: syntax
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tags: [syntax, projections, field-access, pattern-matching]
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order: 12
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---
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# Projections and Field Access
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Enso provides multiple ways for users to access data from their types. It has
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the old functional stalwart of pattern matching, but it also has an inbuilt
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notion of accessors based on lenses (field projections).
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<!-- MarkdownTOC levels="2,3" autolink="true" -->
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- [Pattern Matching](#pattern-matching)
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- [The Underscore in Pattern Matching](#the-underscore-in-pattern-matching)
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- [Projections](#projections)
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<!-- /MarkdownTOC -->
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## Pattern Matching
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Pattern matching in Enso works similarly to as you would expect in various other
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functional languages. Typing information is _always_ refined in the branches of
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a case expression, which interacts well with dependent typing and type-term
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unification. There are a few main ways you can pattern match:
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1. **Positional Matching:** Matching on the scrutinee by structure. This works
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both for atoms and typesets (for typesets it is a subsumption judgement).
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```ruby
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type Vector a
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V2 x:a y:a
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V3 x:a y:a z:a
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v = Vector.V3 x y z
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case v of
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Vector.V3 x y z -> print x
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```
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2. **Type Matching:** Matching purely by the types involved, and not matching
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on structure.
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```ruby
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case v of
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Vector.V3 -> print v.x
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```
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3. **Name Matching on Labels:** Matching on the labels defined within a type
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for both atoms and typesets, with renaming.
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```ruby
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case v of
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Vector.V3 {x y} -> print x
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{x} -> print x
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```
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4. **Naming Scrutinees in Branches:** Ascribing a name of a scrutinee is done
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using the standard typing judgement. This works due to the type-term
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unification present in Enso.
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```ruby
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case _ of
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v : Vector.V3 -> print v,x
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```
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> The actionables for this section :
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>
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> - Refine the syntax for the name-based case.
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> - Provide code examples for why the renaming use-case is important (e.g. cases
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> where there are clashing field names).
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> - Function-resolution matching.
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### The Underscore in Pattern Matching
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An underscore `_` passed as an argument to a syntactic pattern does not behave
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like the function argument shorthand. Instead it acts as a positional match that
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is given no name.
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## Projections
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Unlike the simple accessors defined by most programming language, Enso's
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accessors are far more powerful. This is because they are based on lenses.
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- Field accessors are standard lenses. As such, they can be used for both the
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getting and setting of properties.
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- As a lens (e.g. `.field`) is a first-class function, they can be curried and
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passed around like any other function.
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```ruby
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type Engine
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type Combustion
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power: Int
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cylinder_count: Int
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type Electric
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power: Int
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is_blue: Bool
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type Vehicle
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type Car
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color: String
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max_speed: Int
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engine: Engine
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type Bike
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color: String
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type Person
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type Cons
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name: String
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vehicle: Vehicle
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main =
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p1 = Person.Cons "Joe" (Vehicle.Car 'pink' 300 (Engine.Combustion 500 8))
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print $ p1.name # -> Joe
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print $ p1.vehicle.color # -> pink
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print $ p1.vehicle.max_speed # -> Some 300
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print $ p1.vehicle.engine.power # -> Some 500
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print $ p1.vehicle.engine.is_blue # -> None
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p1.vehicle.color = 'red' # OK
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p1.vehicle.max_speed = 310 # FAIL: security reasons. Allowing this
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# in Haskell was the worst decision
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# ever. After refactoring it
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# silently does nothing there.
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p2 = p1.vehicle.max_speed ?= 310 # OK
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p3 = p1.vehicle.engine.power = 510 # FAIL
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p4 = p1.vehicle.engine.power ?= 510 # OK
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lens_name = .name
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lens_color = .vehicle.color
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lens_max_speed = .vehicle.max_speed
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lens_power = .vehincle.engine.power
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## Function like usage:
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print $ lens_name p1
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print $ lens_color p1
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print $ lens_max_speed p1
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print $ lens_power p1
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p1 . at lens_name = ... # OK
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```
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> The actionables for this section are:
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>
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> - Fix the example above. It isn't correct.
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