Create initial Haskell basics/operators page

[TrueBoxGuy]

Closes #25.

• Emphasise that operators are infix binary functions
• Explain how to use ordinary function identifiers as operators
• Explain usage of operators in prefix, including partial application
• Define and show examples of finding and using operator fixity
• Show a variety of custom operator and fixity declarations

[TrueBoxGuy]

I'm not sure whether I've focussed enough on terminology, or whether it would get in the way of the lesson.

[TrueBoxGuy]

Here's an image of the page.

[soupi]

This is a pretty quality explanation. I have a few comments:

• In a few places, the word "brackets" ([]) is used instead of "parenthesis" (())

• Consider explicitly mentioning how partially applied operators are translated so the positions are more obvious. e.g.

  • (<=) -> \a b -> a <= b
  • (1 <=) -> \a -> 1 <= a
  • (<= 1) -> \a -> a <= 1

• I would suggest adding a type signature when defining an operator in at least one instance just to make sure it is covered

• Consider using this as an opportunity to discuss ++ as an example of right associativity and why it's faster than left associativity (for that case), and why ++ is slow in general (though this could be out of scope here, up to you)

• I would suggest adding a note that recommends to avoid defining operators because they make code harder to read - names usually give clues regarding what the function does and operators don't do that usually, and maybe suggest to make operators aliases to normal functions

• Consider linking to the operators glossary if it make sense

Unrelated: I'm not a fan of using ligatures and unicode symbols in code. I think this can result in confusion for the reader and make the content less accessible. An example of this is <= being rendered as ≤ in this document. I suggest removing ligatures and unicode from the content. @Kleidukos

[Kleidukos]

Agreed to disable ligatures

[TrueBoxGuy]

@Kleidukos do you have any plans on how we should link to FP complete? Can I just put it at the beginning of the lesson?

[Kleidukos]

@TrueBoxGuy Michael allowed us to display FPCo content on Haskell School under condition of referencing the original content on the FPCo website