[1,2,3] is [Int] or [] Int (if we take the numbers to be Ints for simplicity). It is the [] type constructor applied to the Int type. On Tue, Jan 5, 2016 at 5:59 AM Olumide <50295@web.de> wrote:
On 01/01/2016 19:41, Alexander Berntsen wrote:
Can you please give an example of [] used as a type constructor? Whenever you write a list type, e.g. [Int], you are using [] as a type constructor. The fact that you can write [Int] instead of '[] Int' is simply syntax sugar.
We can imagine the simple function that returns the first element of a list, if there is one.
head :: [a] -> Maybe a head [] = Nothing head (x:xs) = Just x
Here we use [] both on type and term level. On type level we use it to mean a list of 'a's, and on term level we use it to mean the empty list.
Out of curiosity, is [] defined as type constructor _and_ term level at the library level or in the language/compiler? (BTW, google tells me "term-level" has a special meaning that I do not yet know.)
Note: I've deferred reading your explanation of kinds because (1) I am still trying to get to grips with Functors and (2) the section on kinds is just two pages away from where I am in the book. So if these questions are answered by kinds please let me know.
Thanks for your patience,
- Olumide
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