Re: [Haskell-cafe] Defining types (newbie)
John Fouhy wrote: But if I want to combine tcEqOne and tcGtThree I run into type problems, because one of them uses Strings and the other Integers. Yep. The observation you can make here is that TC is really a fancy way to write a function that takes some parameters and returns a Bool. So And and Or don't really care what types their parameters have, so long as they return Bools when evaluated. If you could get rid of the type variable on the left, you'd be set, because then And and Or would each take ThingCompares of any type. And you can! See section 7.3.4 of the Hugs manual for existential types: http://cvs.haskell.org/Hugs/pages/hugsman/exts.html#sect7.3.4 This will let you move your type variable a into the TC branch alone as forall a. TC ..., so it will not, er, infect the rest of the branches of the type. Then And and Or will neither care, nor be able to find out, about what TC really has inside, which is as you might wish. b ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Defining types (newbie)
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Defining types (newbie)
On 14/03/07, Bryan O'Sullivan [EMAIL PROTECTED] wrote: 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) Hi Bryan, I actually had to write: data ThingCompare a = TC (a - a - Bool) (Thing - a) runTC :: ThingCompare a - Thing - Thing - Bool runTC (TC compare extract) x y = compare (extract x) (extract y) But thanks for your help; it set me back on the right track. I think things are slowly starting to come back to me :-) -- John. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Defining types (newbie)
On 14/03/07, Bryan O'Sullivan [EMAIL PROTECTED] wrote:
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)
...ok, I'm stuck again. I'm still paralleling How to write a
financial contract, so I'd now like to combine ThingCompare.
This is my code:
---
data Thing = Thing { field_one :: String, field_two :: String,
field_three :: Integer }
t1 :: Thing
t1 = Thing { field_one = foo, field_two = bar, field_three = 17 }
t2 :: Thing
t2 = Thing { field_one = foo, field_two = baz, field_three = 13 }
type BooleanOp a = a - a - Bool
type Field a = Thing - a
data ThingCompare a = TC (BooleanOp a) (Field a)
| And (ThingCompare a) (ThingCompare a)
| Or (ThingCompare a) (ThingCompare a)
and :: ThingCompare a - ThingCompare a - ThingCompare a
and tc1 tc2 = And tc1 tc2
or :: ThingCompare a - ThingCompare a - ThingCompare a
or tc1 tc2 = Or tc1 tc2
runTC :: ThingCompare a - Thing - Thing - Bool
runTC (TC compare extract) x y = compare (extract x) (extract y)
runTC (And tc1 tc2) x y = (runTC tc1 x y) (runTC tc2 x y)
runTC (Or tc1 tc2) x y = (runTC tc1 x y) || (runTC tc2 x y)
tcEqOne = TC (==) field_one
tcEqTwo = TC (==) field_two
tcGtThree = TC () field_three
--
So I can happily write, for example,
tcEqBoth = and tcEqOne tcEqTwo
runTC tcEqBoth t1 t2
and hugs will tell me False.
But if I want to combine tcEqOne and tcGtThree I run into type
problems, because one of them uses Strings and the other Integers. I
want to break the type dependence between the arguments of And; can
I do this? If I type:
data ThingCompare a = TC (BooleanOp a) (Field a)
| And (ThingCompare b) (ThingCompare c)
| Or (ThingCompare b) (ThingCompare c)
hugs complains that it doesn't know what b and c are.
--
John.
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Re: [Haskell-cafe] Defining types (newbie)
John Fouhy: But if I want to combine tcEqOne and tcGtThree I run into type problems, because one of them uses Strings and the other Integers. I want to break the type dependence between the arguments of And; can I do this? Try this. You'll also need to change ThingCompare a to ThingCompare in all your function type signatures. data ThingCompare = forall a. TC (BooleanOp a) (Field a) | And ThingCompare ThingCompare | Or ThingCompare ThingCompare You might also like to read my post from a few minutes ago: http://www.haskell.org/pipermail/haskell-cafe/2007-March/023465.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Defining types (newbie)
I said: Try this. You'll also need to change ThingCompare a to ThingCompare in all your function type signatures. data ThingCompare = forall a. TC (BooleanOp a) (Field a) | And ThingCompare ThingCompare | Or ThingCompare ThingCompare Sorry. For that to work, you would need -fglasgow-exts, which presumes you are using GHC. I just noticed from your post that you are using Hugs. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
