Multiple aggregates in a single pass, using Purescript

Aggregation.purs

-- This is based on ideas from the excellent article "Beautiful Aggregations
-- with Haskell" by Evan Borden: https://tech.freckle.com/2017/09/22/aggregations/
module Aggregation where

import Prelude

import Data.Foldable (foldMap)
import Data.Monoid.Additive (Additive(..))
import Data.Newtype (un)

type Person = { name :: String, occupation :: String, age :: Int }

rows :: Array Person
rows = [ { name: "Bob", occupation: "Dentist", age: 34 }
       , { name: "Tim", occupation: "Nurse", age: 25 }
       , { name: "Ann", occupation: "Programmer", age: 47 }
       , { name: "Mae", occupation: "Astronaut", age: 38 }
       , { name: "Ann", occupation: "Truck driver", age: 55 }
       ]

-- | Produce some summary information from a given list of people in a single pass.
-- |
-- | > summary rows
-- | { annCount: 2, occupations: ["Dentist","Nurse","Programmer","Astronaut","Truck driver"], sumOfAges: 199 }
-- |
-- | Note that this could be written more concisely/generally, but I'm trying to be explicit.
summary :: Array Person -> { occupations :: Array String, sumOfAges :: Int, annCount :: Int }
summary people =
  -- foldMap works like a map/reduce, where each value in a collection is put into a
  -- monoidal value, and those "wrapped" values are combined together into a single
  -- value by the particular monoid that is in use.
  unpack (foldMap step people)
  where
    -- Return a monoidal result for each folded item. It just so happens
    -- that a record whose fields are all monoids is also a monoid, so this function
    -- just returns a record. These records get combined together into a single record
    -- by having their corresponding field values successively combined monoidally.
    step { name, occupation, age } =
      { occupations: pure occupation
      , sumOfAges: pure age
      , annCount: if name == "Ann" then pure 1 else mempty }

    -- Unpack the combined monoidal accumulator, which just involves unpacking
    -- individual fields
    unpack { occupations, sumOfAges, annCount } =
      { occupations, sumOfAges: un Additive sumOfAges, annCount: un Additive annCount }

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