How to define a nested lists
haskell, types
Solution
Instead of a file containing this:
data List a = Elem a | List [List a]
(List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]])
Try a file containing this:
data List a = Elem a | List [List a]
sampleListValue = List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]]
By the way, a very similar type is also available in the standard libraries (I know you don't want to use this as it's a learning exercise, but keep in mind that it's available).
Problem
Trying to solve the Exercise 7 at this page I want to define the data type to write some value like: ``` (List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]]) ``` Nested list, any length, any depth. I tried with this code: ``` data List a = Elem a | List [List a] ``` but it doesn't compile: ``` Parse error: naked expression at top level ``` How can it be done?