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Today's Topics:
1. Re: Lifting over record syntax (Anthony Clayden)
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Message: 1
Date: Wed, 31 Oct 2018 22:52:21 +1300
From: Anthony Clayden <anthony_clay...@clear.net.nz>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Lifting over record syntax
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<CAM7nRYShhpw9Upj=semm0snsd3fmtuxavmocpzqt6ad+uc2...@mail.gmail.com>
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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
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