On Tue, 30 Oct 2018 at 4:20 PM, Anthony Clayden wrote:
data Person = Person {name::String, age::Int} deriving Show
Now, I can create maybe-people like in applicative style:
Person <$> Just "John Doe" <*> Nothing
... field labels for building records only work in very restricted syntactic positions, ...
To tease out that remark a little: Data constructor `Person` is first-class; we could go person' = Person person' <$> Just "John Doe" <*> Nothing But the following two are nothing like equivalent; so record syntax is not even referentially transparent: Person{ name = "Jane Roe", age = 37 } -- builds a Person record person'{ name = "Jane Roe", age = 37 } In the second, the token preceding the `{ ... }` is not a data constructor (because it starts lower case), so is taken to be a variable/expression denoting a value of type `Person`; and this is datatype update syntax. Why of type `Person`? Because field labels `name` and `age` come from there, and under H98 records, they can be associated only with a single type. Then `person'` is the wrong type, and you'll get a type error. Although both look like a name (of function type) adjacent to a term { in braces}, neither is function application. AntC