[Haskell-cafe] Structural typing of records in Haskell?

[David Thomas]

In a sense, Haskell does have structurally typed records - we call them
tuples.

type Rec1 = (Int, Bool)
type Rec2 = (Int, Bool)

Now you can pass either to foo :: Rec1 -> Bool


On Sun, Jan 12, 2014 at 5:00 PM, Cary Cherng <ccherng at gmail.com> wrote:

> Are there statically typed languages that treat records with
> structural typing, either imperative or functional?
>
> Why should records not be structurally typed in Haskell? From what I
> understand, in the below foo cannot take a Rec2 even though Rec1 and
> Rec2 are essentially the same.
>
> data Rec1 = Rec1 { a :: Int, b :: Bool}
> data Rec2 = Rec2 { a :: Int, b :: Bool}
> foo :: Rec1 -> Bool
>
> Rec1 and Rec2 could be in totally different code libraries. I've read
> that preventing Rec2 being used in foo is good for the type safety in
> that Rec1 and Rec2 are likely intended to have semantically different
> meanings and allowing interchangeability breaks this.
>
> But then why is map structurally typed. map takes an argument of type
> a -> b and suppose some other higher order function bar also takes an
> argument of type a -> b. Should map instead have the below type which
> prevents a function of type a -> b semantically intended for bar from
> being accidentally used in map.
>
> newtype Mapper a b = Mapper { fn :: a -> b }
> map :: Mapper a b -> [a] -> [b]
> map _ [] = []
> map f (x:xs) = (fn f) x : map f xs
>
> If there is a mechanism that prevents something of type Rec2 from
> accidentally being used in foo, then why shouldn't there be something
> analogous that prevents something of type a -> b (meant for bar) from
> accidentally being used in map?
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20140113/86f8841d/attachment.html>