F# - Flatten List/Range - f#

Question

I'm new to F# and am wondering how I would go about flattening a list.
Essentially in the database I store a record with a min_age and max_age range (this is a fictitious example for the sake of brevity - i am not agist!). My fields look something like the following:
id,
cost,
savings,
min_age,
max_age
I essentially have an F# class that acts as a one-to-one mapping with this table - i.e. all properties are mapped exactly to the database fields.
What I would like to do is flatten this range. So, instead of a list containing items like this:
saving_id = 1, cost = 100, savings = 20, min_age = 20, max_age = 26
saving_id = 2, cost = 110, savings = 10, min_age = 27, max_age = 31
I would like a list containing items like this:
saving_id = 1, cost = 100, savings = 20, age = 20
saving_id = 1, cost = 100, savings = 20, age = 21
etc.
saving_id = 2, cost = 110, savings = 10, age = 27
saving_id = 2, cost = 110, savings = 10, age = 28
etc.
Is there any in-built mechanism to flatten a list in this manner and/or does anyone know how to achieve this?
Thanks in advance,
JP

Answer 1

You might want to use Seq.collect. It concatenates sequences together, so in your case, you can map a function over your input that splits a single age range record to a sequence of age records and use Seq.collect to glue them together.
For example:
type myRecord =
{ saving_id: int;
cost: int;
savings: int;
min_age: int;
max_age: int }
type resultRecord =
{ saving_id: int;
cost: int;
savings: int;
age: int }
let records =
[ { saving_id = 1; cost = 100; savings = 20; min_age = 20; max_age = 26 }
{ saving_id = 2; cost = 110; savings = 10; min_age = 27; max_age = 31 } ]
let splitRecord (r:myRecord) =
seq { for ageCounter in r.min_age .. r.max_age ->
{ saving_id = r.saving_id;
cost = r.cost;
savings = r.savings;
age = ageCounter }
}
let ageRanges = records |> Seq.collect splitRecord
Edit: you can also use a sequence generator with yield!
let thisAlsoWorks =
seq { for r in records do yield! splitRecord r }

Answer 2

Agreeing with cfern's answer, but was wondering if this might benefit from seeing another "built-in" function used. Here's an alternative version of the splitRecord function that shows the library call for unfolding a sequence. No gain here other than having an example for Seq.unfold.
let splitRecord (r:myRecord) =
Seq.unfold (fun curr_age ->
if curr_age <= r.max_age then
Some({ saving_id = r.saving_id;
cost = r.cost;
savings = r.savings;
age = curr_age } ,
curr_age + 1)
else None)
r.min_age