Question

I have piece of code like this in my Haskell program:

...
start <- getCurrentTime
...
end <- getCurrentTime
...
delta = end `diffUTCTime` start
...

After this, delta has type NominalDiffTime. As written in the documentation, this type in inherited from Num, and I want to print it with 3 (for example) places after decimal dot. However, using something like printf "%.3f" delta doesn’t work: No instance for (PrintfArg NominalDiffTime).

How to do this correctly?

Answer 1

You can convert it to a PrintfArg type to print it in the desired format, e.g.

printf "%.3f" (realToFrac delta :: Double)

does it.