F# Easy · Lesson 1 · On the house

Sequences and laziness

seq expressions, Seq module, infinite sequences, and caching. Lesson 1 is free — on the house.

Lessons · F# Easy · Lesson 1 of 10 · On the house

Sequences and laziness

seq expressions, Seq module, infinite sequences, and caching.

Illustration for F# — Lesson 1 is on the house.

seq<'T> (an alias for IEnumerable<'T>) is lazy: items are computed as they're pulled.

Sequence expressions: seq { for i in 1 .. 3 do yield i * i }; yield! splices another sequence in.

Infinite sequences are fine if you limit them: Seq.initInfinite id |> Seq.take 5.

A seq re-runs its work each time it's enumerated. Seq.cache memoizes, or materialize with Seq.toList.

Use List when you need the whole collection now, Seq for streaming or large/infinite data.

seq

let squares = seq { for i in 1 .. 5 do yield i * i }
printfn "%A" (Seq.toList squares)

let naturals = Seq.initInfinite (fun i -> i + 1)
naturals |> Seq.filter (fun n -> n % 7 = 0) |> Seq.take 3 |> Seq.toList |> printfn "%A"

let mutable runs = 0
let noisy = seq { runs <- runs + 1; yield 1; yield 2 }
noisy |> Seq.sum |> ignore
noisy |> Seq.sum |> ignore
printfn "runs=%d" runs
let cached = Seq.cache noisy
cached |> Seq.sum |> ignore
cached |> Seq.sum |> ignore
printfn "runs=%d" runs
let both = seq { yield 0; yield! [1; 2] }
printfn "%A" (List.ofSeq both)

Match this when you type.

Quiz

When are a seq's elements computed?

At compile time
Once at startup
At definition
When enumerated

Quiz

What does yield! do inside seq { }?

Splices in another sequence
Throws
Yields and stops
Yields lazily twice

Quiz

How many times does noisy run after two sums without caching?

Quiz

What does Seq.cache do?

Memoizes results after first enumeration
Makes it infinite
Sorts
Runs in parallel

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