[Haskell-beginners] data, records and functions

[Peter Hall]

> It's first time I see function type (and where is definition?) in record
> syntax. Can somebody explain this?

There's no definition, it's a parameter to the constructor, so the function
can be anything. Taking a much simpler example, you'll be familiar with, if
you do:

   data Foo a = Foo a

then the first argument to the Foo constructor also doesn't have a
definition. But when you use it to construct a value, then you provide one:

   myFoo = Foo 3

Likewise, when you construct an Ord value, you supply a function as the
value for the 'less' parameter:

   numOrd = Ord { less = (<) }

or you could use a different function for a different purpose:

   listLengthOrd = Ord { less = \ a b => length a < length b }



Hope that helps,

Peter


On 20 February 2013 21:18, Emanuel Koczwara <poczta at emanuelkoczwara.pl>wrote:

> Hi,
>
> I'm watching Philip Wadler "Faith, Evolution, and Programming
> Languages" (very cool by the way):
> http://www.youtube.com/watch?v=8frGknO8rIg
>
> At 00:24:14 there is strange thing on the slide:
>
> data Ord a = { less :: a -> a -> Bool }
>
> It's first time I see function type (and where is definition?) in record
> syntax. Can somebody explain this?
>
> Emanuel
>
>
>
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