18 Aug
2018
18 Aug
'18
5:36 a.m.
Hello Trent, On Sat, Aug 18, 2018 at 02:13:25AM -0700, trent shipley wrote:
Why does Haskell so often seem to treat [] as a general null.
For example I know 0 : 1 : [] gives [0, 1].
But shouldn't it produce a type fault in a consistent world?
Int:Int:List isn't properly a list. It mixes types.
`:` is not syntactic sugar, but a data constructor and behaves like one! λ> :type (:) (:) :: a -> [a] -> [a] "Give me an `a` and a list of `a`, I will return a list." The `empty list` ([]) is polymorphic: λ> :t [] [] :: [a] (it could be an empty list of strings, of ints, of dromedaries), so `3:[]` is well typed. Note that `3:[]:4` will not type-check and that to build a list, you *have* to start with a `[]`.