Evan Klitzke wrote:
I'm writing code with hslogger, and I'm finding myself frequently writing things like this:
infoM $ printf "%s saw %s with %s" (show x) (show y) (show z)
Indeed writing these `show' is tedious. Fortunately, we can get rid of them, still remaining in Haskell98. The following file defines the polyvariadic generic printf: http://okmij.org/ftp/typed-formatting/GenPrintF.hs It is like polyvariadic show with the printf formatting string. It handles values of any type for which there is a Show instance. Two instances of the class SPrintF are all we ever need. Here are the tests: t1 = unR $ printf "Hi there" -- "Hi there" t2 = unR $ printf "Hi %s!" "there" -- "Hi there!" t3 = unR $ printf "The value of %s is %s" "x" 3 -- "The value of x is 3" t4 = unR $ printf "The value of %s is %s" "x" [5] -- "The value of x is [5]" The appearance of unR is only because of Haskell98 (with flexible instances, we can get rid of that). On the other hand, your code post-processes the result of formatting with infoM, so the newtype unwrapping should not matter at all. Still, the code is a bit unsatisfactory because of the appearances of "error" in pr_aux functions. The errors like passing too many or too few arguments to printf (as demanded by the format specification) are caught only at run-time. We can certainly do better.