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
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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