I need help understanding the following statement:
let dictionary = [ for i in 1..4 -> i, true ] |> Map.ofSeq
Specifically, I am confused about the rules regarding the use of brackets.
What does the use of brackets really mean in F#?
What type of collection/set do brackets enclose? (i.e. array, seq, list, etc.)
Can any expression that returns a collection be used within brackets?
NOTE
I know nothing about F#. So please forgive my ignorance.
These are the used brackets in F#
()
(1, 2) // Tuple (separator is a comma)
() // empty tuple, called unit
[]
[ 1; 2 ] // List
[] // List.empty
[| 1; 2 |] // Array
[||] // Array.empty
{}
seq { yield 1; yield 2 } // Sequence
Seq.empty // empty Sequence, not {}
async { return 1 } // Computation Expressions, e.g. async
type record = // Record type definition
{ Name : string
Age : int
}
<>
type A<'T> = A of 'T
Types can easily be composed
let composition =
async { return
[|
[ ("A",1); ("B",2) ]
[ ("C",3) ]
|]
}
// val composition : Async<(string * int) list []>
Just break it up.
let step1 = [ for i in 1..4 -> i, true ]
let step2 = step1 |> Map.ofSeq
Then you may be able to read the "signatures" of each step:
val step1 : (int * bool) list = [(1, true); (2, true); (3, true); (4, true)]
val step2 : Map<int,bool> = map [(1, true); (2, true); (3, true); (4, true)]
step1 is then a list so the brackets are for list (list expression).
Inside the list there is a tuple (algebraic data type, product type) of int and bool (int * bool) (note the * for 'product').
step2 is a Map, created from a sequence, and a list is also a sequence so that is the reason why that works (or lists supports the sequence interface). ofSeq should be the total giveaway I would guess.
Using Map.ofList would probably make it less confusing, but its equivalent
let step1 = [ for i in 1..4 -> i, true ]
let step2 = step1 |> Map.ofList
val step1 : (int * bool) list = [(1, true); (2, true); (3, true); (4, true)]
val step2 : Map<int,bool> = map [(1, true); (2, true); (3, true); (4, true)]
You may benefit from reading some documentation like:
https://msdn.microsoft.com/en-us/library/dd233230.aspx
Now send me my certificate from this course of yours... ;-)
[ sequence-expression ]
Creates a List
Other types of brackets create different types of collections eg. seq { ... } or \[| sequence-expression |\] for sequences (IEnumerable<T>) or arrays respectively.
Can any expression that returns a collection be used within brackets?
Sort of. There is a whole load of support for creating various collections (eg. yield value) including also merging sub-collections (yield! expression).
Really too much for a quick answer. The links about into the F# Reference will show you the scope of this.
Related
In OPL (Optimization Programming Language), we have a data structure name tuple. OPL tuple correspond to Record in F#. Here is how it is defined:
tuple Point {
int x;
int y;
};
Like in F#, we can access field by using dot notation
int x = p.x;
We can group tuples in a Set:
{Point} points = {<1,2>, <2,3>};
A difference is that like in database systems, tuple structures can be associated with keys. Tuple keys enable to access data organized in tuples using a set of unique identifiers. In the following example, the nurse tuple is declared with the key name of type string.
tuple nurse {
key string name;
int seniority;
int qualification;
int payRate;
}
{ nurse } nurses = …;
The nice thing about key, is that we can initialize an array this way
int NumberOfChild [n in nurses] = 0;
and accessing a value by using only the key:
NumberOfChild[<"Isabelle">]=20;
leaving out the fields with no keys. This is equivalent to:
NumberOfChild[<"Isabelle",3,1,16>]=20;
Also, using key means that there will be no two tuples with the same key. Like primary key in database.
Question is: Does some type like this exist in F#? Record with key?
My goal: I would like to define a node structure with many attribute. And load a graph structure by only giving the key of the node and not the entire Record since I would load the graph from a database.
type Node = {
nodeKey : int;
nodeName : string;
nodeAttribute1 : string;
nodeAttribute2 : string }
let Graph = [
(1, 2);
(1, 3);
(2, 4);
(3, 4) ]
Where the int in the graph tuple represent nodeKey.
I would like to do operation using the graph but accessing the node information using the key only.
OPL Grammar
No, there's no such language-level concept. All record fields are created equal, so to speak.
That doesn't preclude you from:
synthesizing a key for a record based on one or more field values,
using such a key as a key in a Map that would hold your records or any additional values.
So you can have something like this:
type Nurse = { name: string; seniority: int; qualification: int; payRate: int }
let nurses = [ { name = "Isabelle"; seniority = 3; qualification = 1; payRate = 16 } ]
let numberOfChildren =
[ "Isabelle", 20 ]
|> Map.ofSeq
let nursesWithNumberOfChildren =
[ for nurse in nurses do
match numberOfChildren |> Map.tryFind nurse.name with
| Some children -> yield nurse, children
| None -> yield nurse, 0 ]
Using similar approach you can separate your graph and node data - store only keys in the graph and maintain a mapping that goes from keys to full node records.
//If I read data from a database, I would receive the data in the following form:
type XYZ = {X:int;
Y:string;
Z:float}
let recordsXYZ = [{X=1;Y="A";Z=1.0};{X=2;Y="b";Z=1.0};{X=3;Y="A";Z=1.0}]
//I can create a map this way
let mapXYZ1=recordsXYZ|>Seq.groupBy (fun a ->a.X)|>Map.ofSeq
//But I don't want a Map<int,seq<XYZ>>
//This is what I want
let mapXYZ2=recordsXYZ|>Seq.map (fun a -> (a.X,{X=a.X;Y=a.Y;Z=a.Z}))|>Map.ofSeq
//Or maybe this is cleaner but this need to define another type
type YZ = {Y:string;
Z:float}
let mapXYZ3=recordsXYZ|>Seq.map (fun a -> (a.X,{Y=a.Y;Z=a.Z}))|>Map.ofSeq
If I understand correctly, your best bet is simply a cleaner alternative to Seq.groupBy for your purposes. Here is the core of it, in one line:
let inline project projection value = projection value, value
Given a simple helper function, not specific to XYZ
let projectToMap projection values = values |> Seq.map (project projection) |> Map.ofSeq
it becomes trivial to cleanly create maps of XYZ from any "key":
let mappedByX = xyzs |> projectToMap (fun { X=x } -> x) // Map<int, XYZ>
let mappedByY = xyzs |> projectToMap (fun { Y=y } -> y) // Map<string, XYZ>
let mappedByZY = xyzs |> projectToMap (fun { Y=y; Z=z } -> z, y) // Map<float*string, XYZ>
Online Demo
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
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>>();;
This actually doesn't cause an issue when it's defined all together. That is, select the entire block that you posted and send it to FSI in one go. I get this:
val divideIntoEquivalenceClasses :
('T -> 'key) -> seq<'T> -> seq<'key * seq<'T>> when 'key : equality
However, if you type these individually into FSI then as John Palmer says there is not enough information in that isolated line for the interpreter to determine the type constraints. John's suggestions will work, but the original code is doing it correctly - defining the variable and using it in the same scope so that the types can be inferred.
for me this code is really ugly, even if this is safe, It looks more similar to imperative languages than to functional lang.
I agree completely – it's slightly tangential to your direct question, but I think a more idiomatic (functional) approach would be:
let divideIntoEquivalenceClasses keyf seq =
(System.Collections.Generic.Dictionary(), seq)
||> Seq.fold (fun dict v ->
let key = keyf v
match dict.TryGetValue key with
| false, _ -> dict.Add (key, ResizeArray(Seq.singleton v))
| _, prev -> prev.Add v
dict)
|> Seq.map (function KeyValue (k, v) -> k, Seq.readonly v)
This allows sufficient type inference to obviate the need for your question in the first place.
The workarounds proposed by the other answers are all good. Just to clarify based on your latest updates, let's consider two blocks of code:
let empties = Array.create 100 []
as opposed to:
let empties = Array.create 100 []
empties.[0] <- [1]
In the second case, the compiler can infer that empties : int list [], because we are inserting an int list into the array in the second line, which constrains the element type.
It sounds like you'd like the compiler to infer a generic value empties : 'a list [] in the first case, but this would be unsound. Consider what would happen if the compiler did that and we then entered the following two lines in another batch:
empties.[0] <- [1] // treat 'a list [] as int list []
List.iter (printfn "%s") empties.[0] // treat 'a list [] as string list []
Each of these lines unifies the generic type parameter 'a with a different concrete type (int and string). Either of these unifications is fine in isolation, but they are incompatible with each other and would result in treating the int value 1 inserted by the first line as a string when the second line is executed, which is clearly a violation of type safety.
Contrast this with an empty list, which really is generic:
let empty = []
Then in this case, the compiler does infer empty : 'a list, because it's safe to treat empty as a list of different types in different locations in your code without ever impacting type safety:
let l1 : int list = empty
let l2 : string list = empty
let l3 = 'a' :: empty
In the case where you make empties the return value of a generic function:
let empties() = Array.create 100 []
it is again safe to infer a generic type, since if we try our problematic scenario from before:
empties().[0] <- [1]
List.iter (printfn "%s") (empties().[0])
we are creating a new array on each line, so the types can be different without breaking the type system.
Hopefully this helps explain the reasons behind the limitation a bit more.
Is there a shorter way of creating an IDictionary<_,obj>, possibly without boxing every value? This is what I have.
let values =
[ "a", box 1
"b", box "foo"
"c", box true ]
|> dict
Dictionary<_,obj>.Add can be called without boxing, but I couldn't figure out a way to use it that's shorter than what I have.
I'm hoping for something other than defining a boxing operator.
EDIT
Based on Brian's suggestion, here's one way to do it, but it has its own problems.
let values =
Seq.zip ["a"; "b"; "c"] ([1; "foo"; true] : obj list) |> dict
Here's a solution, following kvb's suggestion (probably the most concise, and clearest, so far):
let inline (=>) a b = a, box b
let values =
[ "a" => 1
"b" => "foo"
"c" => true ]
|> dict
Here's the slickest thing I was able to whip up. It has more characters than your boxing version, but possibly feels a little less dirty. Note that the ^ is right-associative (it's the string concat operator inherited from ocaml), which lets it work like ::, and it has stronger precedence than ,, which is why the parenthesis are needed around the tuples.
let inline (^+) (x1:'a,x2:'b) (xl:('a*obj) list) =
(x1,box x2)::xl
let values =
("a", 1) ^+ ("b", "foo") ^+ ("c", true) ^+ []
|> dict
I had a similar problem in FsSql and I just tucked away boxing in a function:
let inline T (a,b) = a, box b
let values = dict [T("a",1); T("b","foo"); T("c",true)]
Here's another "solution" which is inspired from Brian's suggestion but it uses reflection so there is a time and safety cost.
let unboxPair (pair:obj) =
let ty = pair.GetType()
let x = ty.GetProperty("Item1").GetValue(pair,null) :?> string
let y = ty.GetProperty("Item2").GetValue(pair,null)
x,y
let unboxPairs (pairs:obj list) =
pairs |> List.map unboxPair
let values =
unboxPairs
["a", 1
"b", "foo"
"c", true]
|> dict
A variation of Stephen's idea:
open System
open System.Collections.Generic
type Dictionary<'a,'b> with
member this.Add([<ParamArray>] args:obj[]) =
match args.Length with
| n when n % 2 = 0 ->
for i in 1..2..(n-1) do
this.Add(unbox args.[i-1], unbox args.[i])
| _ -> invalidArg "args" "even number of elements required"
let d = Dictionary<string,obj>()
d.Add(
"a", 1,
"b", "foo",
"c", true
)
Yet another solution, simply define a bunch of overloaded extension members on Dictionary<'a,'b>:
open System.Collections.Generic
type Dictionary<'a,'b> with
member this.Add(x1,y1,x2,y2) =
this.Add(x1,y1)
this.Add(x2,y2)
member this.Add(x1,y1,x2,y2,x3,y3) =
this.Add(x1,y1,x2,y2)
this.Add(x3,y3)
member this.Add(x1,y1,x2,y2,x3,y3,x4,y4) =
this.Add(x1,y1,x2,y2,x3,y3)
this.Add(x4,y4)
member this.Add(x1,y1,x2,y2,x3,y3,x4,y4,x5,y5) =
this.Add(x1,y1,x2,y2,x3,y3,x4,y4)
this.Add(x5,y5)
member this.Add(x1,y1,x2,y2,x3,y3,x4,y4,x5,y5,x6,y6) =
this.Add(x1,y1,x2,y2,x3,y3,x4,y4,x5,y5)
this.Add(x6,y6)
//etc.
let values =
let d = Dictionary<_,obj>()
d.Add("a", 1,
"b", "foo",
"c", true)
d
Of course values here is not immutable like in your question, but I'm sure you could employ the same strategy in that goal.
let v : (string*obj) list = [...]
let values = dict v
Is one way, the type signature on the left of the list literal will auto-upcast each element.
What is the collection initializer syntax in F#? In C# you can write something like:
new Dictionary<string, int>() {
{"One", 1},
{"two", 2}}
How do I do the same thing in F#? I suppose i could roll my own syntax, but seems like there should be a built-in or standard one already.
To elaborate a bit on collection initialization in F#, here are a few examples:
read-only dictionary
dict [ (1, "a"); (2, "b"); (3, "c") ]
seq (IEnumerable<T>)
seq { 0 .. 99 }
list
[1; 2; 3; 4; 5]
set
set [1; 2; 3; 4; 5]
array
[| 1; 2; 3; 4; 5 |]
As Jared says, there is no built-in support for this for arbitrary collections. However, the C# code is just syntactic sugar for Add method calls, so you could translate it to:
let coll = MyCollectionType()
["One", 1; "Two", 2] |> Seq.iter coll.Add
If you want to get fancy, you could create an inline definition to streamline this even further:
let inline initCollection s =
let coll = new ^t()
Seq.iter (fun (k,v) -> (^t : (member Add : 'a * 'b -> unit) coll, k, v)) s
coll
let d:System.Collections.Generic.Dictionary<_,_> = initCollection ["One",1; "Two",2]
I don't believe F# has an explicit collection initializer syntax. However it's usually very easy to initialize F# collections. For example
let map = [ ("One", 1); ("Two", 2) ] |> Map.ofSeq
Getting to BCL collections is usually a bit more difficult because they don't always have the handy conversion functions. Dictionary<TKey, TValue> works though because you can use the LINQ method
let map =
let list = [ ("One", 1); ("Two", 2) ]
System.Linq.Enumerable.ToDictionary(list, fst, snd)
You can use the same :
open System.Collections.Generic
Dictionary<int, string>(dict [ (1, "a"); (2, "b"); (3, "c") ])
Cheers.
The lack of a collection initializer is annoying for some XAML-centric APIs like Workflow 4.0 which rely on collection initializers instead of ctors, e.g.
new Sequence { Activities = { WriteLine { Text = "In the sequence!" } } };
In such cases, imperative .Add() is awkward because the value is conceptually declarative even though it's technically mutable/imperative. However, there's no common base class for the set of all activities which declare an Activities child: the "Activities" member is a pattern and not an interface, so you can't just write a normal helper function which adds children to any activity. Fortunately, F# member constraints come to the rescue.
In order to write this:
Sequence() |> add [Sequence(DisplayName="InnerSeq"); WriteLine(Text = InArgument<_>("In the sequence!"))]
You first need to define an inline helper function called "add":
let inline add (children: Activity seq) =
let inline doAdd (activity: ^Activity) : ^Activity when ^Activity : (member get_Activities : unit -> Activity Collection) =
let collection = (^Activity : (member get_Activities : unit -> Activity Collection) (activity))
for child in children do
collection.Add(child)
activity
doAdd
This still isn't quite as nice as the C# syntax but at least it's still declarative. IMHO this is not so much as a fault with F# as with collection-initializer-centric APIs, but at least F# allows a workaround.
Given that the C# collection initializer syntax is syntactic sugar for calling .Add and that implies a mutable collection - I'm not sure you'll see any such syntax in F#. It's initialize all in one go as per JaredPar's answer, or do it manually.
Anyone have a decent example, preferably practical/useful, they could post demonstrating the concept?
(Edit: a small Ocaml FP Koan to start things off)
The Koan of Currying (A koan about food, that is not about food)
A student came to Jacques Garrigue and said, "I do not understand what currying is good for." Jacques replied, "Tell me your favorite meal and your favorite dessert". The puzzled student replied that he liked okonomiyaki and kanten, but while his favorite restaurant served great okonomiyaki, their kanten always gave him a stomach ache the following morning. So Jacques took the student to eat at a restaurant that served okonomiyaki every bit as good as the student's favorite, then took him across town to a shop that made excellent kanten where the student happily applied the remainder of his appetite. The student was sated, but he was not enlightened ... until the next morning when he woke up and his stomach felt fine.
My examples will cover using it for the reuse and encapsulation of code. This is fairly obvious once you look at these and should give you a concrete, simple example that you can think of applying in numerous situations.
We want to do a map over a tree. This function could be curried and applied to each node if it needs more then one argument -- since we'd be applying the one at the node as it's final argument. It doesn't have to be curried, but writing another function (assuming this function is being used in other instances with other variables) would be a waste.
type 'a tree = E of 'a | N of 'a * 'a tree * 'a tree
let rec tree_map f tree = match tree with
| N(x,left,right) -> N(f x, tree_map f left, tree_map f right)
| E(x) -> E(f x)
let sample_tree = N(1,E(3),E(4)
let multiply x y = x * y
let sample_tree2 = tree_map (multiply 3) sample_tree
but this is the same as:
let sample_tree2 = tree_map (fun x -> x * 3) sample_tree
So this simple case isn't convincing. It really is though, and powerful once you use the language more and naturally come across these situations. The other example with some code reuse as currying. A recurrence relation to create prime numbers. Awful lot of similarity in there:
let rec f_recurrence f a seed n =
match n with
| a -> seed
| _ -> let prev = f_recurrence f a seed (n-1) in
prev + (f n prev)
let rowland = f_recurrence gcd 1 7
let cloitre = f_recurrence lcm 1 1
let rowland_prime n = (rowland (n+1)) - (rowland n)
let cloitre_prime n = ((cloitre (n+1))/(cloitre n)) - 1
Ok, now rowland and cloitre are curried functions, since they have free variables, and we can get any index of it's sequence without knowing or worrying about f_recurrence.
While the previous examples answered the question, here are two simpler examples of how Currying can be beneficial for F# programming.
open System.IO
let appendFile (fileName : string) (text : string) =
let file = new StreamWriter(fileName, true)
file.WriteLine(text)
file.Close()
// Call it normally
appendFile #"D:\Log.txt" "Processing Event X..."
// If you curry the function, you don't need to keep specifying the
// log file name.
let curriedAppendFile = appendFile #"D:\Log.txt"
// Adds data to "Log.txt"
curriedAppendFile "Processing Event Y..."
And don't forget you can curry the Printf family of function! In the curried version, notice the distinct lack of a lambda.
// Non curried, Prints 1 2 3
List.iter (fun i -> printf "%d " i) [1 .. 3];;
// Curried, Prints 1 2 3
List.iter (printfn "%d ") [1 .. 3];;
Currying describes the process of transforming a function with multiple arguments into a chain of single-argument functions. Example in C#, for a three-argument function:
Func<T1, Func<T2, Func<T3, T4>>> Curry<T1, T2, T3, T4>(Func<T1, T2, T3, T4> f)
{
return a => b => c => f(a, b, c);
}
void UseACurriedFunction()
{
var curryCompare = Curry<string, string, bool, int>(String.Compare);
var a = "SomeString";
var b = "SOMESTRING";
Console.WriteLine(String.Compare(a, b, true));
Console.WriteLine(curryCompare(a)(b)(true));
//partial application
var compareAWithB = curryCompare(a)(b);
Console.WriteLine(compareAWithB(true));
Console.WriteLine(compareAWithB(false));
}
Now, the boolean argument is probably not the argument you'd most likely want to leave open with a partial application. This is one reason why the order of arguments in F# functions can seem a little odd at first. Let's define a different C# curry function:
Func<T3, Func<T2, Func<T1, T4>>> BackwardsCurry<T1, T2, T3, T4>(Func<T1, T2, T3, T4> f)
{
return a => b => c => f(c, b, a);
}
Now, we can do something a little more useful:
void UseADifferentlyCurriedFunction()
{
var curryCompare = BackwardsCurry<string, string, bool, int>(String.Compare);
var caseSensitiveCompare = curryCompare(false);
var caseInsensitiveCompare = curryCompare(true);
var format = Curry<string, string, string, string>(String.Format)("Results of comparing {0} with {1}:");
var strings = new[] {"Hello", "HELLO", "Greetings", "GREETINGS"};
foreach (var s in strings)
{
var caseSensitiveCompareWithS = caseSensitiveCompare(s);
var caseInsensitiveCompareWithS = caseInsensitiveCompare(s);
var formatWithS = format(s);
foreach (var t in strings)
{
Console.WriteLine(formatWithS(t));
Console.WriteLine(caseSensitiveCompareWithS(t));
Console.WriteLine(caseInsensitiveCompareWithS(t));
}
}
}
Why are these examples in C#? Because in F#, function declarations are curried by default. You don't usually need to curry functions; they're already curried. The major exception to this is framework methods and other overloaded functions, which take a tuple containing their multiple arguments. You therefore might want to curry such functions, and, in fact, I came upon this question when I was looking for a library function that would do this. I suppose it is missing (if indeed it is) because it's pretty trivial to implement:
let curry f a b c = f(a, b, c)
//overload resolution failure: there are two overloads with three arguments.
//let curryCompare = curry String.Compare
//This one might be more useful; it works because there's only one 3-argument overload
let backCurry f a b c = f(c, b, a)
let intParse = backCurry Int32.Parse
let intParseCurrentCultureAnyStyle = intParse CultureInfo.CurrentCulture NumberStyles.Any
let myInt = intParseCurrentCultureAnyStyle "23"
let myOtherInt = intParseCurrentCultureAnyStyle "42"
To get around the failure with String.Compare, since as far as I can tell there's no way to specify which 3-argument overload to pick, you can use a non-general solution:
let curryCompare s1 s2 (b:bool) = String.Compare(s1, s2, b)
let backwardsCurryCompare (b:bool) s1 s2 = String.Compare(s1, s2, b)
I won't go into detail about the uses of partial function application in F# because the other answers have covered that already.
It's a fairly simple process. Take a function, bind one of its arguments and return a new function. For example:
let concatStrings left right = left + right
let makeCommandPrompt= appendString "c:\> "
Now by currying the simple concatStrings function, you can easily add a DOS style command prompt to the front of any string! Really useful!
Okay, not really. A more useful case I find is when I want to have a make a function that returns me data in a stream like manner.
let readDWORD array i = array[i] | array[i + 1] << 8 | array[i + 2] << 16 |
array[i + 3] << 24 //I've actually used this function in Python.
The convenient part about it is that rather than creating an entire class for this sort of thing, calling the constructor, calling obj.readDWORD(), you just have a function that can't be mutated out from under you.
You know you can map a function over a list? For example, mapping a function to add one to each element of a list:
> List.map ((+) 1) [1; 2; 3];;
val it : int list = [2; 3; 4]
This is actually already using currying because the (+) operator was used to create a function to add one to its argument but you can squeeze a little more out of this example by altering it to map the same function of a list of lists:
> List.map (List.map ((+) 1)) [[1; 2]; [3]];;
val it : int list = [[2; 3]; [4]]
Without currying you could not partially apply these functions and would have to write something like this instead:
> List.map((fun xs -> List.map((fun n -> n + 1), xs)), [[1; 2]; [3]]);;
val it : int list = [[2; 3]; [4]]
I gave a good example of simulating currying in C# on my blog. The gist is that you can create a function that is closed over a parameter (in my example create a function for calculating the sales tax closed over the value of a given municipality)out of an existing multi-parameter function.
What is appealing here is instead of having to make a separate function specifically for calculating sales tax in Cook County, you can create (and reuse) the function dynamically at runtime.