Monoid

A Monoid in Haskell is a data type with a lawful Monoid instance, that is:

class Semigroup a => Monoid a where
    mempty :: a
    mappend :: a -> a -> a
    mappend = (<>)

Monoids are data types that can be instantiated empty and be smashed together without losing any information. A common example is and [a] (for any a), where mempty=[] and mappend=(++). Monoids are always Semigroups as well.

2020-09-22

Haskell^ᛦ
- Monoid^ᛦ
Semigroup
- A Monoid without an identity element. In Haskell:
Superpowered keyword args in Haskell
- fetchPostArgs looks a lot like a Monoid, and we can certainly frame it that way, with a little bit of finagling:
- Using Monoids gives us better composition of common operations in many cases. I might argue that taking this step is unnecessary for this particular example, but it is at least helpful for illustrative purposes and might come in handy depending on your use case.
- The conceptual reason for this is that fetchPostsForUser has no default behavior without a UserId, and we need default behavior for the Monoid to remain sensible.