Haskell: print TextEncoding
Haskell newbie here.
$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1
While trying to debug weird locale-related bug in third-party Haskell program, I'm trying to print default encoding:
import System.IO
main = do
print localeEncoding
But it fails:
$ ghc -o printlocale main.hs
main.hs:4:2:
No instance for (Show TextEncoding)
arising from a use of `print' at main.hs:4:2-21
Possible fix: add an instance declaration for (Show TextEncoding)
In the expression: print localeEncoding
In the expression: do { print localeEncoding }
In the definition of `main': main = do { print localeEncoding }
My google-fu is failing me. What am I missing?
To print a value of some type in Haskell, the type must be an instance of the Show class.
localeEncoding :: TextEncoding
and TextEncoding is not an instance of Show.
The TextEncoding type is actually an existential type storing the methods for encoding and decoding:
data TextEncoding
= forall dstate estate . TextEncoding {
mkTextDecoder :: IO (TextDecoder dstate),
mkTextEncoder :: IO (TextEncoder estate)
}
Since these are functions, there's no sensible way to show them. The current localeEncoding is determined using iconv, via the C function nl_langinfo.
So, TextEncoding as such is not a showable type, so you cannot print it. However, you can construct new values of this type, via mkTextEncoding. Eg to create a utf8 environment:
mkTextEncoding "UTF-8"
We might consider a feature request to store the representation of the locale with the TextEncoding, so this label could be printed. However, that's currently not possible.
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