Un programa escrito en Haskell, que al ejecutarse...

[Casimiro Notevi]

¿Un programa escrito en Haskell, que al ejecutarse imprime un programa en Perl, que al ejecutarse imprime un programa en Python, que al ejecutarse imprime un programa en Ruby, que al ejecutarse imprime un programa en C, que al ejecutarse imprime un programa en Java, que al ejecutarse imprime el programa original?.
Sí, es como para que te duela la cabeza, aunque el código, que podéis ver después del salto, es de lo más sencillo. Este tipo de programas que se imprimen a sí mismos como salida (aunque en este caso, con una peculiaridad extra) se conocen con el nombre de quines, en honor al filósofo estadounidense Willard Van Orman Quine.

import Data.List

langs = [Haskell, Perl, Python, Ruby, C, Java]

data Languages = Haskell | Ruby | Perl | C | Python | Java

sequenceFromString Haskell s = "map toEnum[" ++ (intercalate "," $
    map (\c -> show (fromEnum c)) s) ++ "]"
sequenceFromString Perl s    = (intercalate "," $
    map (\c -> "chr(" ++ show (fromEnum c) ++ ")") s)
sequenceFromString Python s  = (intercalate "+" $
    map (\c -> "chr(" ++ show (fromEnum c) ++ ")") s)
sequenceFromString Ruby s    = (intercalate "+" $
    map (\c -> show (fromEnum c) ++ ".chr") s)
sequenceFromString C s       = concatMap
    (\c -> "putchar(" ++ show (fromEnum c) ++ ");") s
sequenceFromString Java s    = concatMap
    (\c -> "o.write(" ++ show (fromEnum c) ++ ");") s

paramList' Haskell = intercalate " " . map (\n -> "a" ++ show n)
paramList' C       = intercalate "," . map (\n -> "char *a" ++ show n)
paramList' Python  = intercalate "," . map (\n -> "a" ++ show n)
paramList' Ruby    = intercalate "," . map (\n -> "a" ++ show n)
paramList' Java    = intercalate "," . map (\n -> "String a" ++ show n)

paramList Perl    _ = ""
paramList lang n = paramList' lang [0..n-1]

driver l args = defn l ++ intercalate (divider l) args ++ endDefn l

divider C       = "\",\""
divider Perl    = "','"
divider Ruby    = "\",\""
divider Python  = "\",\""
divider Haskell = "\" \""
divider Java    = "\",\""

defn C       = "main(){q(\""
defn Perl    = "&q('"
defn Python  = "q(\""
defn Ruby    = "q(\""
defn Haskell = "main=q \""
defn Java    = "public static void main(String[]args){q(\""

endDefn C       = "\");}"
endDefn Perl    = "')"
endDefn Python  = "\")"
endDefn Ruby    = "\")"
endDefn Haskell = "\""
endDefn Java    = "\");}}"

arg Haskell n = "a" ++ show n
arg Perl n    = "$_[" ++ show n ++ "]"
arg C n       = "printf(a" ++ show n ++ ");"
arg Python n  = "a" ++ show n
arg Ruby n    = "a" ++ show n
arg Java n    = "o.print(a" ++ show n ++ ");"

argDivide Haskell l = "++" ++ sequenceFromString Haskell (divider l) ++ "++"
argDivide Perl l    = ","    ++ sequenceFromString Perl (divider l) ++ ","
argDivide C l       = sequenceFromString C (divider l)
argDivide Python l  = "+" ++ sequenceFromString Python (divider l) ++ "+"
argDivide Ruby l    = "+" ++ sequenceFromString Ruby (divider l) ++ "+"
argDivide Java l    = sequenceFromString Java (divider l)

argList lang1 lang2 n = intercalate (argDivide lang1 lang2) $
    map (arg lang1) ([1..n-1] ++ [0])

fromTo Haskell l n = "q " ++ paramList Haskell n ++ "=putStrLn$a0++" ++
    sequenceFromString Haskell ("\n" ++ defn l) ++ "++" ++
    argList Haskell l n ++ "++" ++ sequenceFromString Haskell (endDefn l)
fromTo Perl    l n = "sub q {" ++ "print $_[0]," ++
    sequenceFromString Perl ("\n" ++ defn l) ++ "," ++ argList Perl l n ++ "," ++
    sequenceFromString Perl (endDefn l ++ "\n") ++ "}"
fromTo Python  l n = "def q(" ++ paramList Python n ++
    "): print a0+" ++ sequenceFromString Python ("\n" ++ defn l) ++
    "+" ++ argList Python l n ++ "+" ++ sequenceFromString Python (endDefn l)
fromTo Ruby    l n = "def q(" ++ paramList Ruby n ++
    ") print a0+" ++ sequenceFromString Ruby ("\n" ++ defn l) ++
    "+" ++ argList Ruby l n ++ "+" ++ sequenceFromString Ruby (endDefn l ++ "\n") ++ " end"
fromTo C       l n = "q(" ++ paramList C n ++ "){" ++ "printf(a0);" ++
    sequenceFromString C ("\n" ++ defn l) ++ argList C l n ++
    sequenceFromString C (endDefn l ++ "\n") ++ "}"
fromTo Java    l n = "public class quine{public static void q(" ++
    paramList Java n ++ "){java.io.PrintStream o=System.out;o.print(a0);" ++
    sequenceFromString Java ("\n" ++ defn l) ++ argList Java l n ++
    sequenceFromString Java (endDefn l ++ "\n") ++ "}"

main = do
    let n = length langs
    let langs' = cycle langs
    putStrLn $ fromTo (head langs') (head (tail langs')) n
    putStrLn $ driver (head langs') $ zipWith (\lang1 lang2 -> fromTo lang1 lang2 n)
        (take n (tail langs')) (tail (tail langs'))

Leído en mundogeek