Records.md

Records

Record literals are surrounded by braces, as in JavaScript:

author :: { name :: String, interests :: Array String }
author =
    { name: "Phil"
    , interests: ["Functional Programming", "JavaScript"]
    }

Fields of records can be accessed using a dot, followed by the label of the field to access:

> author.name
"Phil"

> author.interests
["Functional Programming","JavaScript"]

Kinds

{ ... } is just syntactic sugar for the Record type constructor, so { language :: String } is the same as Record ( language :: String ).

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.

( 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 ).

Extending Records

It is possible to define an extensible record

type Lang l = { language :: String | l }

that can then be extended like:

type Language = Lang ( country :: String )

The Language type synonym would then be equivalent to { language :: String, country :: String }. Note that parentheses must be used for the extension, since l has to be a row kind not a record type.

Wildcards

Record literals with wildcards can be used to create a function that produces the record instead:

{ foo: _, bar: _ }

is equivalent to:

\foo bar -> { foo: foo, bar: bar }

Record Update

PureScript also provides a record update syntax similar to Haskell's:

setX :: Number -> Point -> Point
setX val point = point { x = val }

This can be used to update nested records:

setPersonPostcode :: PostCode -> Person -> Person
setPersonPostcode pc p = p { address { postCode = pc } }