[Haskell-cafe] Re: Learning about Programming Languages (specifically Haskell)

[Rafael Gustavo da Cunha Pereira Pinto]

If you are running from GHCi, just type run 100 at the prompt..

If you intend to compile it, you have to add

main = print $ run 100

The compiler adds a call to main::IO (), which is intended to be the main
entry point of your code.

We need to add print, as run has type

run::Int->[Door]

so run 100 has type [Door].

print is

print::(Show a) => a -> IO ()

The IO () stands for an empty IO monad, which is the black magic of haskell,
intended to separate pure code from I/O side-effects...




On Mon, May 3, 2010 at 06:31, Samuel Williams <
space.ship.traveller at gmail.com> wrote:

> Also, one more thing - if someone could write some comments to go along
> with the source code that explain what it is doing, that would be really
> helpful. I can see the general structure, but I don't know the ins and outs
> of Haskell. If someone could augment the example with comments explaining
> what the functions do that would be great!
>
> data *Door* = *Open* | *Closed* deriving *Show*
>
>
> toggle *Open*   = *Closed*
> toggle *Closed* = *Open*
>
>
> pass k = zipWith ($) (cycle $ replicate k *id* ++ [toggle])
>
>
> run n = foldl (flip pass) (replicate n *Closed*) [0..n]
>
> Do I need to add run 100 to the end of the example for it to actually do
> something?
>
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>


-- 
Rafael Gustavo da Cunha Pereira Pinto
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