Re: Records in Haskell

28 Feb 2012

      J. Garrett Morris  writes:
...
Has x f has no semantic content besides type x having an f field; Ord
has (at least in the programmer's mind, even if the language can't check
it) meaning beyond the simple presence of a compare function.
/g
Note that under both SORF and DORF, there are three arguments to the `Has` 
class. The third is specifically to spec or constrain the type of the result.

A decl:

    data Rec a = Ord a => Rec{ flda :: a }

Expects:

    flda :: (r{ flda :: a }, Ord a) => r -> a

Where the {...} in the constraint is sugar for the Has class. (DORF and SORF 
differ slightly in how that's implemented.)

AntC