Add a shorthand/literal syntax for creating Maps

[baronfel]

Add a shorthand/literal syntax for creating Maps [12536532]

Submitted by Brad Collins on 3/1/2016 12:00:00 AM
3 votes on UserVoice prior to migration

A shorthand/literal syntax or computation expression for building Maps would be nice—something like the following:
let m = map ["a",97; "b",98; "c",99]
The shorthand for dictionaries ...
let d = dict ["a",97; "b",98; "c",99]
... is nice, but while Maps implement IDictionary, dictionaries do not play well with Map module functions. One must inevitably convert a dictionary to a Map or create a Map with Map.ofArray, .ofList, or .ofSeq.

Original UserVoice Submission
Archived Uservoice Comments

[xperiandri]

I think it must be type agnostic for any type supporting KeyValuePair
Something similar to C# like

let map = [
    ["key1"] = "value1"
    ["key1"] = "value1"
]

let map : ImmutableDictionary<string, string> = [
    ["key1"] = "value1"
    ["key1"] = "value1"
]

So that if type supports constructor or AddRange(KeyValuePair<'TKey,'TValue> seq) it will work

[jwosty]

Wouldn't let map = Map.ofSeq work for the usage that OP provided?

@xperiandri Good idea -- if we're going to make this a language suggestion, it would need to be a meaningful extension to the language like you're suggesting. However, your proposed syntax would be ambiguous (could be a bool list). Perhaps something more like the following (though I don't like the usage of naked square brackets, as there's still some ambiguity there between the list syntax):

let map = [
    "key1" -> "value1"
    "key2" -> "value2"
]

Side note: you hack something together (for maps in particular) that syntactically gets you pretty close in today's F# (weird, but fun):

let map = Map.ofSeq
let (=>) key value = key, value // since you can't have a -> operator
let xs = map [
    "key1" => 42
    "key2" => 43
]

[xperiandri]

Then maybe

let map = [
    ["key1"] <- "value1"
    ["key1"] <- "value1"
]

or

let map = [
    .["key1"] <- "value1"
    .["key1"] <- "value1"
]

[xperiandri]

if we're going to make this a language suggestion

There is a similar discussion about collections #619

I suppose these can be combined into a singles suggestion that will allow initializing collections and maps using let collection : CollectionType = [] syntax

[dsyme]

I'm going to close this. It's little known but you can already do

Map [ (1,3); (3,4) ]

You can also do

Set [ 1;3;4 ]

So if anything we should consider deprecating set [1;3;4] to have uniformity

[xperiandri]

Is it a real collection initializer and not creating list and then a set or map?

[dsyme]

As things stand, it creates the intermediate list

[Lanayx]

This issue might we relevant to type-directed collection initialization #1086. I'd like to see the type-directed resolutions for Map, Dictionary, Sets, IDictionary and classes that inherit from Dictionary and IDictionary as well.

[abelbraaksma]

Change from completed -> not planned

[brianrourkeboll]

Just for reference, it is possible to make a Map<_, _> builder that works with no intermediate collections: https://github.com/brianrourkeboll/FSharp.Collections.Builders.

As to whether such a thing should be in the core library: that depends on whether type-direction for […] were ever going to support maps or dictionaries as a target. C# does seem on track to add dictionary initializer syntax, for what it's worth.

[abelbraaksma]

Yes, good points. Perhaps we should create a separate issue to collect what we currently know about type-directed collection initializers. I believe there are some other issues lying around that aren't specific to Maps.