[Haskell-cafe] Implicit Multi Threading in Switch Case

[Isaac Elliott]

Hey Yotam,

Pattern matching clauses evaluate from left to right. You can get the
behavour you're after by swapping the arguments in the tuple:

case (stop, infi) of
  (Just _, Nothing) -> 1
  (_, _) -> 2

Evaluates to 2

On Tue, 20 Jun. 2017, 7:29 pm Yotam Ohad, <yotam2206 at gmail.com> wrote:

> Hi,
> After reading "Push-Pull Functional Reactive Programming
> <https://pdfs.semanticscholar.org/fb7a/879d639641341e025197b40afad9e21f0ce5.pdf>"
> I had an idea about deciding which, of two events, comes first. Instead of
> using forkIO, I tried something like the following:
>
> infi :: Maybe Int
> infi = infi
>
> stop :: Maybe Int
> stop = Nothing
>
> test :: Int
> test = case (infi, stop) of
>     (Nothing, Just _) -> 1
>     (_, _) -> 2
>
> Here, infi is an action that never ends, and stop a function that ends
> immediately. I thought that the compiler would see that stop evaluates
> immediately to Nothing and thus will return 2, but it tries to evaluate
> infi and get stuck.
>
> I think it happens because I am using a tuple to hold both values (but not
> really sure about it). Do you know a way to make this arrangement work?
>
> Yotam
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