On 12-03-11 01:36 PM, Chris Smith wrote:
On Sun, Mar 11, 2012 at 11:22 AM, Mario Blažević<blama...@acanac.net> wrote:
No, idP does terminate once it consumes its input. Your idP>> p first
reproduces the complete input, and then runs p with empty input.
This is just not true. idP consumes input forever, and (idP>> p) =
idP, for all pipes p.
If it is composed with another pipe that terminates, then yes, the
*composite* pipe can terminate, so for example ((q>+> idP)>> p) may
actually do something with p. But to get that effect, you need to
compose before the monadic bind... so for example (q>+> (idP>> p)) =
(q>+> idP) = q. Yes, q can be exhausted, but when it is, idP will
await input, which will immediately terminate the (idP>> p) pipe,
producing the result from q, and ignoring p entirely.
Sorry. I was describing the way it's done in SCC, and I assumed
that pipes and pipes-core behaved the same. But GHCi says you're right:
> :{
| runPipe ((fromList [1, 2, 3] >> return [])
| >+> (idP >> fromList [4, 5] >> return [])
| >+> consume)
| :}
[1,2,3]
May I enquire what was the reason for the non-termination of idP?
Why was it not defined as 'forP yield' instead? The following command
runs the way I expected.
> :{
| runPipe ((fromList [1, 2, 3] >> return [])
| >+> (forP yield >> fromList [4, 5] >> return [])
| >+> consume)
| :}
[1,2,3,4,5]
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