FS-1135 - Random functions for collections

RFCs/FS-1135-random-functions-for-collections.md

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+# F# RFC FS-1135 - Random functions for collections (List, Array, Seq)
+
+The design suggestion [Add shuffle, sample etc. methods for lists, arrays etc](https://github.com/fsharp/fslang-suggestions/issues/508) has been marked "approved in principle".
+
+This RFC covers the detailed proposal for this suggestion.
+
+- [x] [Suggestion](https://github.com/fsharp/fslang-suggestions/issues/508)
+- [x] Approved in principle
+- [ ] [Implementation]() (no implementation yet)
+- [ ] Design Review Meeting(s) with @dsyme and others invitees
+- [Discussion](https://github.com/fsharp/fslang-design/discussions/731)
+
+# Summary
+
+This feature extends collections apis with functions for random sampling and shuffling built-in fsharp collections.
+
+# Motivation
+
+This feature is motivated by the following use cases:
+ - Using F# for data science and machine learning (like building a neural network), where data shuffling plays important role
+ - Building games, where random sampling is used for generating random levels, random decks, etc.
+ - Building simulations, where random sampling is used for generating random input data
+
+# Detailed design
+
+### General
+
+The following general rules are applied to all functions
+ - New function should be implemented in List, Array, Seq modules
+ - All functions should not mutate the input collection
+ - All functions should have a variant with a [Random](https://learn.microsoft.com/en-us/dotnet/api/system.random) parameter
+ - Shared thread-safe Random instance should be used for all basic functions.
+
+### Shuffle
+
+Shuffle function should returned a new shuffled collection of the same collection type.
+
+Two functions should be added to each module.
+
+```fsharp
+// Array module
+val shuffle: 'T[] -> 'T[]
+val shuffleRand: Random -> 'T[] -> 'T[]
+// List module
+val shuffle: 'T list -> 'T list
+val shuffleRand: Random -> 'T list -> 'T list
+// Seq module
+val shuffle: 'T seq -> 'T seq
+val shuffleRand: Random -> 'T seq -> 'T seq
+```
+[ArgumentNullException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentnullexception) should be raised if collection is null
+
+Example:
+```fsharp
+let allPlayers = [ "Alice"; "Bob"; "Charlie"; "Dave" ]
+let round1Order = allPlayers |> List.shuffle // [ "Charlie"; "Dave"; "Alice"; "Bob" ]
+```
+
+### Choice
+
+Choice function should returned a random element from the collection.
+
+Two functions should be added to each module.
+
+```fsharp
+// Array module
+val choice: 'T[] -> 'T
+val choiceRand: Random -> 'T[] -> 'T
+// List module
+val choice: 'T list -> 'T
+val choiceRand: Random -> 'T list -> 'T
+// Seq module
+val choice: 'T seq -> 'T
+val choiceRand: Random -> 'T seq -> 'T
+```
+[ArgumentNullException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentnullexception) should be raised if collection is null
+
+[InvalidOperationException](https://learn.microsoft.com/en-us/dotnet/api/system.invalidoperationexception) should be raised if collection is empty
+
+Example:
+```fsharp
+let allPlayers = [ "Alice"; "Bob"; "Charlie"; "Dave" ]
+let round1Order = allPlayers |> List.choice // "Charlie"
+```
+
+### Choices
+
+Choices should select N elements from input collection in random order, once element is taken it can be selected again.
+
+Two functions should be added to each module.
+
+```fsharp
+// Array module
+val choices: int -> 'T[] -> 'T[]
+val choicesRand: Random -> int -> 'T[] -> 'T[]
+// List module
+val choices: int -> 'T list -> 'T list
+val choicesRand: Random -> int -> 'T list -> 'T list
+// Seq module
+val choices: int -> 'T seq -> 'T seq
+val choicesRand: Random -> int -> 'T seq -> 'T seq
+```
+[ArgumentNullException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentnullexception) should be raised if collection is null
+
+[ArgumentOutOfRangeException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception) should be raised if N is negative
+
+[InvalidOperationException](https://learn.microsoft.com/en-us/dotnet/api/system.invalidoperationexception) should be raised if collection is empty
+
+Example:
+```fsharp
+let allPlayers = [ "Alice"; "Bob"; "Charlie"; "Dave" ]
+let round1Order = allPlayers |> List.choices 3 // ["Bob", "Dave", "Bob"]
+```
+
+### Sample
+
+Sample should select N elements from input collection in random order, once element is taken it won't be selected again. N can't be greater than collection length
+
+Two functions should be added to each module.
+
+```fsharp
+// Array module
+val sample: int -> 'T[] -> 'T[]
+val sampleRand: Random -> int -> 'T[] -> 'T[]
+// List module
+val sample: int -> 'T list -> 'T list
+val sampleRand: Random -> int -> 'T list -> 'T list
+// Seq module
+val sample: int -> 'T seq -> 'T seq
+val sampleRand: Random -> int -> 'T seq -> 'T seq
+```
+[ArgumentNullException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentnullexception) should be raised if collection is null
+
+[ArgumentOutOfRangeException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception) should be raised if N is greater than collection length or negative
+
+[InvalidOperationException](https://learn.microsoft.com/en-us/dotnet/api/system.invalidoperationexception) should be raised if collection is empty
+
+Example:
+```fsharp
+let allPlayers = [ "Alice"; "Bob"; "Charlie"; "Dave" ]
+let round1Order = allPlayers |> List.sample 3 // ["Charlie", "Dave", "Alice"]
+```
+
+# Drawbacks
+
+System.Random interface has added some new methods in [.NET 8](https://learn.microsoft.com/en-us/dotnet/core/whats-new/dotnet-8#methods-for-working-with-randomness), where naming is a bit different. More new methods can eventually be added in future .NET versions.
+
+# Alternatives
+
+Use online snippets, or provide a nuget package.
+
+# Compatibility
+
+Please address all necessary compatibility questions:
+
+* Is this a breaking change? **No, unless user defined those extensions themselves**
+* What happens when previous versions of the F# compiler encounter this design addition as source code? **Should be fine**
+* What happens when previous versions of the F# compiler encounter this design addition in compiled binaries? **Should be fine**
+* If this is a change or extension to FSharp.Core, what happens when previous versions of the F# compiler encounter this construct? **Will work as usual**
+
+# Pragmatics
+
+## Diagnostics
+
+Please list the reasonable expectations for diagnostics for misuse of this feature. **I don't see a way to misuse it**
+
+## Tooling
+
+Please list the reasonable expectations for tooling for this feature, including any of these:
+
+* Debugging
+    * Breakpoints/stepping
+    * Expression evaluator
+    * Data displays for locals and hover tips
+* Auto-complete
+* Tooltips
+* Navigation and Go To Definition
+* Colorization
+* Brace/parenthesis matching
+
+**Should work just like for other collections functions**
+
+## Performance
+
+Please list any notable concerns for impact on the performance of compilation and/or generated code
+
+* For existing code **Existing code should not be affected**
+* For the new features **Performance should be respected when implementing this feature, since it can be used in performance-sensitive scenaris**
+
+## Scaling
+
+Algorithmic complexity of the new features should be O(n) or less. 
+
+## Culture-aware formatting/parsing
+
+N/A
+
+# Unresolved questions
+
+It's unclear, if more sophisticated overloads should be added:
+ - Weights parameter for choices function
+ - Counts parameter for sample function
+
+~~In .NET 6 and higher [Random.Shared](https://learn.microsoft.com/en-us/dotnet/api/system.random.shared) is available, but as soon as F# Core only targets standard, it can't use it. Is targeting higher .NET possible (not just for this feature, maybe some others need it)?
+Same question about new [.NET 8 apis](https://learn.microsoft.com/en-us/dotnet/core/whats-new/dotnet-8#methods-for-working-with-randomness), they could be reused in theory~~