Overview

Common F♯ collections

Module	Type	-	BCL Equivalent	Immutable	Structural comparison
Array	'T array	≡	Array<T>	❌	✅
List	'T list	≃	ImmutableList<T>	✅	✅
Seq	seq<'T>	≡	IEnumerable<T>	✅	✅
Set	Set<'T>	≃	ImmutableHashSet<T>	✅	✅
Map	Map<'K, 'V>	≃	ImmutableDictionary<K,V>	✅	✅
❌	dict	≡	IDictionary<K,V>	☑️ ❗	❌
❌	readOnlyDict	≡	IReadOnlyDictionary<K,V>	☑️	❌
❌	ResizeArray	≡	List<T>	❌	❌

Functions consistency 👍

Common to all 5 modules:

• empty/isEmpty, exists/forall

• find/tryFind, pick/tryPick, contains (containsKey for Map)

• map/iter, filter, fold

Common to Array, List, Seq:

• append/concat, choose, collect

• item, head, last

• take, skip

• ... a hundred functions altogether!

Syntax consistency 👍

Type	Construction	Range	Comprehension📍
Array	[∣ 1; 2 ∣]	[∣ 1..5 ∣]	✅
List	[ 1; 2 ]	[ 1..5 ]	✅
Seq	seq { 1; 2 }	seq { 1..5 }	✅
Set	set [ 1; 2 ]	set [ 1..5 ]	✅

Syntax trap ⚠️

Square brackets [] are used both for:

• Value: instance of a list [ 1; 2 ] (of type int list)

• Type: array int [], e.g. of [| 1; 2 |]

☝ Recommendations

• Distinguish between type vs value ❗

• Write int array rather than int[]

Comprehension

• Purpose: syntactic sugar to construct collection

  • Handy, succinct, powerful

  • Syntax includes for loops, if condition

• Same principle as generators in C♯, JS

  • yield keyword but often optional (since F♯ 4.7)

  • yield! keyword (pronounce "yield bang") ≡ yield* in JS

  • Works for all collections 👍

Examples:

// Multi-line (recommended)
let squares =
    seq { for i in 1 .. 10 do
        yield i * i // 💡 'yield' can be omitted most of the time 👍
    }

// Single line
let squares = seq { for i in 1 .. 10 -> i * i }

// Can contain 'if'
let halfEvens =
    [ for i in [1..10] do
        if (i % 2) = 0 then i / 2 ]  // [1; 2; 3; 4; 5]

// Nested 'for'
let pairs =
    [ for i in [1..3] do
      for j in [1..3] do
        i, j ]              // [(1, 1); (1; 2); (1; 3); (2, 1); ... (3, 3)]

Flattening:

// Multiple items
let twoToNine =
    [ for i in [1; 4; 7] do
        if i > 1 then i
        i + 1
        i + 2 ]  // [2; 3; 4; 5; 6; 7; 8; 9]

// With 'yield! collections'
let oneToSix =
    [ for i in [1; 3; 5] do
        yield! [i; i+1] ]