From 78ff5cede01973c6279c6bad10010a42109040df Mon Sep 17 00:00:00 2001 From: milesfrain Date: Sat, 27 Jun 2020 22:40:01 -0700 Subject: [PATCH 1/3] Add section on record puns and type synonyms for records --- language/Records.md | 42 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 42 insertions(+) diff --git a/language/Records.md b/language/Records.md index 25fa1561..ff9e8f4f 100644 --- a/language/Records.md +++ b/language/Records.md @@ -20,6 +20,15 @@ Fields of records can be accessed using a dot, followed by the label of the fiel ["Functional Programming","JavaScript"] ``` +Type synonyms for record types are created with the `type` keyword: + +```purs +type Point = + { x :: Number + , y :: Number + } +``` + ## Kinds `{ ... }` is just syntactic sugar for the `Record` type constructor, so `{ language :: String }` is the same as `Record ( language :: String )`. @@ -79,6 +88,39 @@ A record update function can also be defined by using an `_` inplace of the reco _ { fieldName = newValue } ``` +## Record Puns + +_Record puns_ enable concise code when record fields have the same name as other values. This feature is useful for constructing records and pattern matching. + +### Constructing + +```purs +origin :: Point +origin = { x, y } +-- origin = { x: x, y: y } -- Equivalent + where + x = 0.0 + y = 0.0 +``` + +### Pattern Matching + +```purs +showPoint :: Point -> String +showPoint { x, y } = show x <> ", " <> show y +-- showPoint { x : x, y : y } = show x <> ", " <> show y -- Equivalent +``` + +### Not for Record Updates + +Note that puns may not be used for record updates. + +```purescript +setX :: Number -> Point -> Point +setX x point = point { x = x } +-- setX x point = point { x } -- Not allowed +``` + ## Field Names Symbols which are illegal value identifiers, such as title-cased identifiers or ones containing spaces, can be used to identify a field by enclosing it in double-quotes: From b77b502c066b6c57408694f7c345a78f344bc030 Mon Sep 17 00:00:00 2001 From: milesfrain Date: Thu, 16 Jul 2020 20:14:05 -0700 Subject: [PATCH 2/3] Add section on Merging Records --- language/Records.md | 26 ++++++++++++++++++++++++++ 1 file changed, 26 insertions(+) diff --git a/language/Records.md b/language/Records.md index ff9e8f4f..9c290d3a 100644 --- a/language/Records.md +++ b/language/Records.md @@ -121,6 +121,32 @@ setX x point = point { x = x } -- setX x point = point { x } -- Not allowed ``` +## Merging Records + +The [`record`](https://pursuit.purescript.org/packages/purescript-record) package enables additional record operations, such as `merge` and `union`. + +Here's an example of using the `disjointUnion` function to add a `z` field to a `Point` record. The `to3d` function merges the original record `p` with another record `{ z }` created from this new `Number`: + +```purescript +import Record (disjointUnion) + +type Point3d + = { x :: Number + , y :: Number + , z :: Number + } + +to3d :: Point -> Number -> Point3d +to3d p z = disjointUnion p { z } + +-- Equivalent to: +to3d p z = + { x: p.x + , y: p.y + , z + } +``` + ## Field Names Symbols which are illegal value identifiers, such as title-cased identifiers or ones containing spaces, can be used to identify a field by enclosing it in double-quotes: From 144193290f6f05e022ec2feb0fc66a72dd285aec Mon Sep 17 00:00:00 2001 From: milesfrain Date: Tue, 29 Dec 2020 13:36:37 -0800 Subject: [PATCH 3/3] Apply review feedback - kinds, builder, formatting --- language/Records.md | 40 +++++++++++++++++++++++++--------------- 1 file changed, 25 insertions(+), 15 deletions(-) diff --git a/language/Records.md b/language/Records.md index 9c290d3a..4bf9b0ee 100644 --- a/language/Records.md +++ b/language/Records.md @@ -31,11 +31,25 @@ type Point = ## Kinds -`{ ... }` is just syntactic sugar for the `Record` type constructor, so `{ language :: String }` is the same as `Record ( language :: String )`. +`{ ... }` is syntactic sugar for the `Record` type constructor. This type constructor is parameterized by a row of types: -The Record type constructor is parameterized by a row of types. In kind notation, `Record` has kind `# Type -> Type`. That is, it takes a row of types to a type. +```purescript +-- these types are equivalent +type Language = { language :: String } +type Language' = Record ( language :: String ) +``` + +A `Record` is constructed from a row type and represents a product type in which all fields in the row type are present. Using kind notation, `Record` has the kind `# Type -> Type` -- that is, it takes a row of types and produces a type. + +Because `( language :: String )` denotes a row of types (and therefore has the kind `# Type`), it can be passed to the `Record` constructor or to the `{ ... }` syntax for `Record` to construct a type: + +```purescript +type LanguageRow = ( language :: String ) -- has kind # Type -`( language :: String )` denotes a row of types (something of kind `# Type`), so it can be passed to `Record` to construct a type, namely `Record ( language :: String )`. +-- these are equivalent +type Language = Record LanguageRow +type Language' = { | LanguageRow } +``` ## Extending Records @@ -121,30 +135,26 @@ setX x point = point { x = x } -- setX x point = point { x } -- Not allowed ``` -## Merging Records +## Further Record Operations -The [`record`](https://pursuit.purescript.org/packages/purescript-record) package enables additional record operations, such as `merge` and `union`. +The [`record`](https://pursuit.purescript.org/packages/purescript-record) package enables additional record operations, such as `merge`, `union`, and efficient ways to create records (see [Record.Builder](https://pursuit.purescript.org/packages/purescript-record/docs/Record.Builder)). Here's an example of using the `disjointUnion` function to add a `z` field to a `Point` record. The `to3d` function merges the original record `p` with another record `{ z }` created from this new `Number`: ```purescript import Record (disjointUnion) -type Point3d - = { x :: Number - , y :: Number - , z :: Number - } +type Point3d = + { x :: Number + , y :: Number + , z :: Number + } to3d :: Point -> Number -> Point3d to3d p z = disjointUnion p { z } -- Equivalent to: -to3d p z = - { x: p.x - , y: p.y - , z - } +to3d p z = { x: p.x, y: p.y, z } ``` ## Field Names