I do not notice this before. "fun ([0, 1] ++ xs) = .." in my code could not be compiled, parse error.
Maybe a small abstract can help, as I once also got confused by that. * First, syntax without operators You can only match on constructors. So, if you have data Test = Test1 String | Test2 Integer | Test3 you can do function (Test1 s) = ... function (Test2 i) = ... function Test3 = ... * Second, syntax with operators Haskell allow constructors made of symbols, but you have to start them with ':', so this is valid: data Test = Test1 String | Integer :** String and then function (Test1 s) = ... function (i :** s) = ... * Third, special syntax Haskell has special syntax for tuples and lists (and something else I forgot?). You can ask information about a name in ghci using ':i <name>', see what it says about (,) and []: data (,) a b = (,) a b data [] a = [] | a : [a] As you can see, (,), [] and : are actually constructors, and you can pattern match on them: function [] = ... function (a:b) = ... function ((:) a b) = ... function (a,b) = ... function ((,) a b) = ... Best, Maurício