varying number of arguments restriction

I'm curious why the following code is invalid (from a language design point of view):

foo :: [(Int, String)] -> String foo [] = "" foo = snd . head

ghc complains: Varying number of arguments for function `foo' I don't understand why this should be invalid? Basically, as I read it, I say, well, for a parameter s, calling foo s: does s match [] ? if so, return "" otherwise, try the next definition of foo (which must match in this case). obviously i can rewrite: foo [] = "" foo s = (snd . head) s but this is uglier. - hal -- Hal Daume III "Computer science is no more about computers | hdaume@isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume