What is Haskell's equivalent of
string str = string.Format("{0} {1}",10,20); // C#
5 Answers
There is a Printf module in GHC.
import Text.Printf
str :: String
str = printf "%d %d" 10 20
however it is probably simpler to just do
str = show 10 ++ " " ++ show 20
6 Comments
Pratik Deoghare
Its not working I got
ERROR filename.hs:3:Cannot justify constraints in explicitly typed binding. I am using WinHugs.
Don Stewart
You should switch to GHC. Hugs is unmaintained, slow, and supports very few of the packages on Hackage. It isn't part of the Haskell Platform specification either.
SwiftsNamesake
Text.Printf.printf returns an IO action (IO ()) when fully applied, not a String.
newacct
@SwiftsNamesake: It also returns
String. It can return any type that is PrintfType.
Kevin Zhu
anything like python's f-string?
|
You could use the format function provided by the text-format-simple package:
import Text.Format
format "{0} {1}" [show 10, show 20]
This function has the signature:
format :: String -> [String] -> String
So all you need is provide your parameters as strings.
Another example:
format "Some {0} believes that 1 + 1 = {1}." ["people",show 10]
3 Comments
Bryan O'Sullivan
I'd recommend /not/ using that package, as it's very constrained in what it can do. I'm working on a package named text-format which is generally more capable, and already quite usable for this purpose.
Lambda Fairy
Dmitry, all the packages you listed save for double-conversion are included in the Haskell Platform. The vast majority of users won't have to install that much. You argument is still valid though.
Jules
@BryanO'Sullivan Thanks for the pointer. I've had a look at your package, and while it looks much more useful than the alternatives (and has a type for the format specifier rather than plain string, which seems like an extremely good idea) unfortunately for me it produces
Data.Text as its output, rather than String. As I need to integrate with existing code that relies on String, this would require an unnecessary conversion for me. Is there an alternative that produces a String output and supports a format specifier that allows you to change the order of fields in the output?Putting answer here in case somebody searching for formatting libraries in Haskell on StackOverflow. There's type-safe and fast formatting library called fmt now. With it you can write code like this:
> "There are "+|n|+" million bicycles in "+|city|+"."
Comments
2 Comments
Julia
This answer does not work for me:
Ambiguous type variable ‘t0’ arising from a use of ‘printf’ prevents the constraint ‘(Text.Printf.PrintfArg t0)’ from being solved.
éclairevoyant
@Julia Works for me, perhaps you forgot to import the module?
Alternatively you could write
unwords [show 10, show 20]
or even
unwords (map show [10, 20])
{0, 10}?