CODEWITHSUNDEEP
haskelltype-conversion
aplavin
aplavin

Reputation: 2229

Print NominalDiffTime in Haskell

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?

Upvotes: 3

Views: 1552

Answers (2)

Aleksandar Dimitrov
Aleksandar Dimitrov

Reputation: 9487

If you want a nicely formatted diff time, you can use System.Time.Utils (renderSecs) together with round

 ghci> import Data.Time.Clock
 ghci> import System.Time.Utils (renderSecs)
 ghci> 
 ghci> let renderS = renderSecs . round :: NominalDiffTime -> String
 ghci> t1 <- getCurrentTime
 ghci> -- wait a little :-P
 ghci> t2 <- getCurrentTime
 ghci> renderS $ t2 `diffUTCTime` t1
 "2m15s"

It will also do days as d, and hours as h. Note that the type signature is mandatory for ghci, because otherwise it'll default to Double, and suddenly you can't feed it NominalDiffTime values.

Upvotes: 6

Daniel Fischer
Daniel Fischer

Reputation: 183868

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

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

does it.

Upvotes: 6

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