Re: [Haskell-cafe] what does @ mean?.....

@ works as an aliasing primitive for the arguments of a function f x@(Just y) = ... using "x" in the body of f is equivalent to use "Just y". Perhaps in this case is not really useful, but in some other cases it saves the effort and space of retyping really long expressions. And what is even more important, in case an error is made when choosing the pattern, you only have to correct it in one place. On Dec 28, 2007 12:05 PM, Nicholls, Mark wrote:

Hello, I wonder if someone could answer the following…

The short question is what does @ mean in

mulNat a b

| a <= b = mulNat' a b b

| otherwise = mulNat' b a a

where

mulNat' x@(S a) y orig

| x == one = y

| otherwise = mulNat' a (addNat orig y) orig

The long version, explaining what everything means is….

here's a definition of multiplication on natural numbers I'm reading

on a blog....

data Nat = Z | S Nat

deriving Show

one :: Nat

one = (S Z)

mulNat :: Nat -> Nat -> Nat

mulNat _ Z = Z

mulNat Z _ = Z

mulNat a b

| a <= b = mulNat' a b b

| otherwise = mulNat' b a a

where

mulNat' x@(S a) y orig

| x == one = y

| otherwise = mulNat' a (addNat orig y) orig

Haskell programmers seem to have a very irritating habit of trying to

be overly concise...which makes learnign the language extremely

hard...this example is actually relatively verbose....but anyway...

Z looks like Zero...S is the successor function...Nat are the

"Natural" numbers.....

mulNat _ Z = Z

mulNat Z _ = Z

translates to...

x * 0 = 0....fine...

0 * x = 0....fine..

mulNat a b

| a <= b = mulNat' a b b

| otherwise = mulNat' b a a

where

mulNat' x@(S a) y orig

| x == one = y

| otherwise = mulNat' a (addNat orig y) orig

is a bit more problematic...

lets take a as 3 and b as 5...

so now we have

mulNat' 3 5 5

but what does the "x@(S a)" mean? in

mulNat' x@(S a) y orig

________________________________

From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Nicholls, Mark Sent: 21 December 2007 17:47 To: David Menendez Cc: Jules Bean; haskell-cafe@haskell.org Subject: RE: [Haskell-cafe] nice simple problem for someone struggling....

Let me resend the code…as it stands….

module Main where

data SquareType numberType = Num numberType => SquareConstructor numberType

class ShapeInterface shape where

area :: Num numberType => shape->numberType

data ShapeType = forall a. ShapeInterface a => ShapeType a

instance (Num a) => ShapeInterface (SquareType a) where

area (SquareConstructor side) = side * side

and the errors are for the instance declaration…….

[1 of 1] Compiling Main ( Main.hs, C:\Documents and Settings\nichom\Haskell\Shapes2\out/Main.o )

Main.hs:71:36:

Couldn't match expected type `numberType' against inferred type `a'

`numberType' is a rigid type variable bound by

the type signature for `area' at Main.hs:38:15

`a' is a rigid type variable bound by

the instance declaration at Main.hs:70:14

In the expression: side * side

In the definition of `area':

area (SquareConstructor side) = side * side

I'm becoming lost in errors I don't comprehend….

What bamboozles me is it seemed such a minor enhancement.

________________________________

From: d4ve.menendez@gmail.com [mailto:d4ve.menendez@gmail.com] On Behalf Of David Menendez Sent: 21 December 2007 17:05 To: Nicholls, Mark Cc: Jules Bean; haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] nice simple problem for someone struggling....

On Dec 21, 2007 11:50 AM, Nicholls, Mark wrote:

Now I have....

module Main where

data SquareType numberType = Num numberType => SquareConstructor numberType

This is a valid declaration, but I don't think it does what you want it to. The constraint on numberType applies only to the data constructor.

That is, given an unknown value of type SquareType a for some a, we do not have enough information to infer Num a.

For your code, you want something like:

instance (Num a) => ShapeInterface (SquareType a) where area (SquareConstructor side) = side * side

-- Dave Menendez <http://www.eyrie.org/~zednenem/ > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe