diff options
Diffstat (limited to 'libraries/base/Debug/Trace.hs')
| -rw-r--r-- | libraries/base/Debug/Trace.hs | 10 |
1 files changed, 8 insertions, 2 deletions
diff --git a/libraries/base/Debug/Trace.hs b/libraries/base/Debug/Trace.hs index 26a19d8f71..653dcab055 100644 --- a/libraries/base/Debug/Trace.hs +++ b/libraries/base/Debug/Trace.hs @@ -150,8 +150,14 @@ traceShowId a = trace (show a) a {-| Like 'trace' but returning unit in an arbitrary 'Applicative' context. Allows -for convenient use in do-notation. Note that the application of 'trace' is not -an action in the 'Applicative' context, as 'traceIO' is in the 'IO' type. +for convenient use in do-notation. + +Note that the application of 'traceM' is not an action in the 'Applicative' +context, as 'traceIO' is in the 'IO' type. While the fresh bindings in the +following example will force the 'traceM' expressions to be reduced every time +the @do@-block is executed, @traceM "not crashed"@ would only be reduced once, +and the message would only be printed once. If your monad is in 'MonadIO', +@liftIO . traceIO@ may be a better option. > ... = do > x <- ... |