[Haskell-cafe] Re: Pattern matching does not work like this?

[Maurí­cio]

>   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