[Haskell-cafe] Defining types (newbie)

[Bryan O'Sullivan]

John Fouhy wrote:

> In Haskell, I envisage writing something like:
> 
> data ThingCompare = TC Op Field

This wants to be a bit more concrete:

data ThingCompare = TC (a -> a -> Bool) (Thing -> a)

so that you could then have something to execute your comparison thingy:

runTC :: ThingCompare -> Thing -> Thing -> Bool
runTC (TC compare extract) a b = compare (extract a) (extract b)

and construct a value of it:

tcEqFirst = TC (==) first_field

so to compare two Things, you'd do something like this:

runTC tcEqFirst a b

	<b