Personal Haskell Learning experience
https://www.quora.com/How-is-polymorphism-used-in-functional-programming-languages
https://wiki.haskell.org/Introduction_to_IO https://www.haskell.org/tutorial/io.html https://stackoverflow.com/questions/12226624/newbie-understanding-main-and-io
https://stackoverflow.com/questions/42988950/let-vs-regular-assignment-in-ghci https://stackoverflow.com/questions/14052093/ghci-let-what-does-it-do
Expression in Haskell
In Haskell, expression is value, function or an actual expression. Haskell is all about mathematical expressions which are modelled after Mathematical notion of lambda calculus.
Foldable type in JavaScript
Haskell String are foldable in a sense that you can concat them. Thus when you do:
Prelude> :type concat
concat :: Foldable t => t [a] -> a
-- Note, Foldable t => t [a] is equivalent to [[a]]. Think Foldable t as some list.Foldable type is less formally known as concatable type. String type is foldable but not Char type. Essential thing here is that concatable returns a same type. So concatenation of two strings produces string. (Dr. Booleans's videos for more details)
Or probably I am wrong. Foldable may not be concatable.
List operator
(!!) is an operator on list to get item at certain index in a list.
Guidelines
It is rarely a good idea to write a partial function. They will cause runtime exception.
Just like if-else, case statements are expressions.