GroupBy using list of records in Elm - elm

Question

What would an Elm function definition look like which takes a list of records, and perform a groupby operation (see example below)? Using a SQL analogy, I'm basically trying to achieve select last, sum(age) from table group by lastin Elm.
[{name= "John", last= "Smith", age= 10}
,{name="Jason", last= "Smith", age= 9}
, {name="Jane", last ="White", age =5}]
return [{last="Smith", age =19},
{last ="White", age =5}]

Answer 1

You could use Dict.Extra.groupBy to perform the initial grouping, then sum the ages by mapping over the list and summing the ages:
import Dict.Extra exposing (groupBy)
familySumAges : List Person -> List Family
familySumAges =
groupBy .last
>> Dict.map (\_ -> List.map .age >> List.sum)
>> Dict.toList
>> List.map (uncurry Family)

Answer 2

The following answer courtesy of #holy-meekrob and #ilias from the Elm Slack channel. https://ellie-app.com/7tqY9w6gNa1/1
module Main exposing (..)
import Dict exposing (..)
import Html exposing (text)
type alias Person =
{ name : String
, last : String
, age : Int
}
type alias Family =
{ last : String
, age : Int
}
people : List Person
people =
[ { name = "John"
, last = "Smith"
, age = 10
}
, { name = "Jason"
, last = "Smith"
, age = 9
}
, { name = "Jane"
, last = "White"
, age = 5
}
]
sumAges : Person -> Dict String Family -> Dict String Family
sumAges person families =
Dict.update
person.last
(\family ->
case family of
Nothing ->
Just { last = person.last, age = person.age }
Just fam ->
Just { last = person.last, age = fam.age + person.age }
)
families
main =
text (toString (List.foldl sumAges Dict.empty people |> Dict.values))

Answer 3

My approach is different than the solution you accepted.
If the list of records is sorted, then we could simply aggregate the age as long as the last name remains unchanged.
Otherwise, we'll need to create a new record for the unseen last name.
import List
SumAgeByLast data = let
sorted = List.sortBy .last data
fst x = List.head x |> Maybe.withDefault {name= "_", last= "_", age= 0}
agg lst p =if (fst lst).last==p.last then {name = "_", last = p.last ,age = (fst lst).age+p.age} else p::lst
in
List.foldl agg [] sorted
We basically iterate over all people p and add them to the first element of the list lst unless we encounter that last name for the first time, then we prepend it p::lst
Test it with
data = [{name= "John", last= "Smith", age= 10},{name="Jason", last= "Smith", age= 9}, {name="Jane", last ="White", age =5}]
SumByLast data
Obviously, this function could be made generic, if we pass .last and .age as parameters