[Haskell-cafe] Any ideas on what this should generate and not generate...?

[Jacco Krijnen]

Hi,

In the first example the traceShows are only called as a side effect of
evaluating the IO expression (traceShow "Bla" $ do traceShow "Bloe" runit)
which happens only once (runit is a top-level binding). I assume that for
the second example GHC lifts (traceShow "Num" print 2) out of the do block
and similarly evaluates it only once.

The traceShow "Bloe" is not lifted out of there because it's under a
lambda. E.g. if you do this instead:

runit :: IO ()
runit = traceShow "Bla" $ do
  traceShow "Num" (print 2)
  traceShow "Bloe" runit

You get

"Bla"
"Num"
2
"Bloe"
2
2
2
2
...


Jacco

2017-06-28 16:16 GMT+01:00 Jurriaan Hage <J.Hage at uu.nl>:

> We had a strange phenomenon while debugging a large compiler.
> This is the smallest example, but we have seen this behavior in our own
> monad.
>
> module TracEx where
>
> import Debug.Trace
>
> runit :: IO ()
> runit = traceShow "Bla" $ do
>   traceShow "Bloe” runit
>
>
> and this generates
> Bla
> Bloe
>
> and then it hangs. Nothing else is generated.
> We are guessing this has to do with optimisations of some kind.
>
> And what about
>
> runit :: IO ()
> runit = traceShow "Bla" $ do
>   n <- traceShow "Num" print 2
>   traceShow "Bloe” runit
>
> Now we get
> Bla
> Num
> 2
> Bloe
> 2
> Bloe
> 2
> …
> No more Bla and no more Num.
>
> Any ideas whether this is a bug or a feature?
>
> Jur and Tibor
>
>
>
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