Question

I’m reading Expert F# book and I found this code

open System.Collections.Generic
let divideIntoEquivalenceClasses keyf seq =
// The dictionary to hold the equivalence classes
  let dict = new Dictionary<'key,ResizeArray<'T>>()
  // Build the groupings
  seq |> Seq.iter (fun v ->
                          let key = keyf v
                          let ok,prev = dict.TryGetValue(key)
                          if ok then prev.Add(v)
                          else let prev = new ResizeArray<'T>()
                             dict.[key] <- prev
                             prev.Add(v))

 dict |> Seq.map (fun group -> group.Key, Seq.readonly group.Value)

and the example use:

> divideIntoEquivalenceClasses (fun n -> n % 3) [ 0 .. 10 ];;
val it : seq<int * seq<int>>
= seq [(0, seq [0; 3; 6; 9]); (1, seq [1; 4; 7; 10]); (2, seq [2; 5; 8])]

first for me this code is really ugly, even if this is safe, It looks more similar to imperative languages than to functional lang..specially compared to clojure. But the problem is not this…I’m having problems with the Dictionary definition

when I type this:

let dict = new Dictionary<'key,ResizeArray<'T>>();;

I get this:

pruebafs2a.fs(32,5): error FS0030: Value restriction. The value 'dict' has been inferred to have generic type
val dict : Dictionary<'_key,ResizeArray<'_T>> when '_key : equality    

Either define 'dict' as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation.

is It ok?…

thanks so much

---

improve question:

Ok I’ve been reading about value restriction and I found this helpfull information

In particular, only function definitions and simple immutable data
expressions are automatically generalized

…ok..this explains why

let dict = new Dictionary<'key,ResizeArray<'T>>();;

doesn’t work…and show 4 different techniques, although in my opinion they only resolve the error but aren’t solutions for use generic code:

Technique 1: Constrain Values to Be Nongeneric

 let empties : int list [] = Array.create 100 []

Technique 3: Add Dummy Arguments to Generic Functions When Necessary

let empties () = Array.create 100 []
let intEmpties : int list [] = empties()   

Technique 4: Add Explicit Type Arguments When Necessary (similar to tec 3)

let emptyLists = Seq.init 100 (fun _ -> [])
> emptyLists<int>;;
val it : seq<int list> = seq [[]; []; []; []; ...]

—– and the only one than let me use real generic code ——
Technique 2: Ensure Generic Functions Have Explicit Arguments

let mapFirst = List.map fst //doesn't work
let mapFirst inp = List.map fst inp

Ok, in 3 of 4 techniques I need resolve the generic code before can work with this…now…returning to book example…when the compile knows the value for ‘key and ‘T

let dict = new Dictionary<‘key,ResizeArray<‘T>>()

in the scope the code is very generic for let key be any type, the same happen with ‘T

and the biggest dummy question is :

when I enclose the code in a function (technique 3):

let empties = Array.create 100 [] //doesn't work
let empties () = Array.create 100 []
val empties : unit -> 'a list []

I need define the type before begin use it
let intEmpties : int list [] = empties() 

for me (admittedly I’m a little dummy with static type languages) this is not real generic because it can’t infer the type when I use it, I need define the type and then pass values (not define its type based in the passed values) exist other way define type without be so explicit..

thanks so much..really appreciate any help

Answer 1

This line

let dict = new Dictionary<'key,ResizeArray<'T>>();;

fails because when you type the ;; the compiler doesn’t know what 'key and 'T are. As the error message states you need to add a type annotation, or allow the compiler to infer the type by using it later or make it a function

Examples

Type annotation change

let dict = new Dictionary<int,ResizeArray<int>>();;

Using types later

let dict = new Dictionary<'key,ResizeArray<'T>>()
dict.[1] <- 2

using a function

let dict() = new Dictionary<'key,ResizeArray<'T>>();;