F# language - hints for newbie [closed] - f#

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Looks like here in StackOveflow there is a group of F# enthusiasts.
I'd like to know better this language, so, apart from the functional programming theory, can you point me to the better starting points to start using the F# language? I mean, tutorials, how-tos, but first of all working samples to have the chance to start doing something and enjoy the language.
Thanks a lot
Andrea

Not to whore myself horribly but I wrote a couple F# overview posts on my blog here and here. Chris Smith (guy on the F# team at MS) has an article called 'F# in 20 minutes' - part 1 and part 2.
Note you have to be careful as the latest CTP of F# (version 1.9.6.0) has some seriously breaking changes compared to previous versions, so some examples/tutorials out there might not work without modification.
Here's a quick run-down of some cool stuff, maybe I can give you a few hints here myself which are clearly very brief and probably not great but hopefully gives you something to play with!:-
First note - most examples on the internet will assume 'lightweight syntax' is turned on. To achieve this use the following line of code:-
#light
This prevents you from having to insert certain keywords that are present for OCaml compatibility and also having to terminate each line with semicolons. Note that using this syntax means indentation defines scope. This will become clear in later examples, all of which rely on lightweight syntax being switched on.
If you're using the interactive mode you have to terminate all statements with double semi-colons, for example:-
> #light;;
> let f x y = x + y;;
val f : int -> int -> int
> f 1 2;;
val it : int = 3
Note that interactive mode returns a 'val' result after each line. This gives important information about the definitions we are making, for example 'val f : int -> int -> int' indicates that a function which takes two ints returns an int.
Note that only in interactive do we need to terminate lines with semi-colons, when actually defining F# code we are free of that :-)
You define functions using the 'let' keyword. This is probably the most important keyword in all of F# and you'll be using it a lot. For example:-
let sumStuff x y = x + y
let sumStuffTuple (x, y) = x + y
We can call these functions thus:-
sumStuff 1 2
3
sumStuffTuple (1, 2)
3
Note there are two different ways of defining functions here - you can either separate parameters by whitespace or specify parameters in 'tuples' (i.e. values in parentheses separated by commas). The difference is that we can use 'partial function application' to obtain functions which take less than the required parameters using the first approach, and not with the second. E.g.:-
let sumStuff1 = sumStuff 1
sumStuff 2
3
Note we are obtaining a function from the expression 'sumStuff 1'. When we can pass around functions just as easily as data that is referred to as the language having 'first class functions', this is a fundamental part of any functional language such as F#.
Pattern matching is pretty darn cool, it's basically like a switch statement on steroids (yeah I nicked that phrase from another F#-ist :-). You can do stuff like:-
let someThing x =
match x with
| 0 -> "zero"
| 1 -> "one"
| 2 -> "two"
| x when x < 0 -> "negative = " + x.ToString()
| _ when x%2 = 0 -> "greater than two but even"
| _ -> "greater than two but odd"
Note we use the '_' symbol when we want to match on something but the expression we are returning does not depend on the input.
We can abbreviate pattern matching using if, elif, and else statements as required:-
let negEvenOdd x = if x < 0 then "neg" elif x % 2 = 0 then "even" else "odd"
F# lists (which are implemented as linked lists underneath) can be manipulated thus:-
let l1 = [1;2;3]
l1.[0]
1
let l2 = [1 .. 10]
List.length l2
10
let squares = [for i in 1..10 -> i * i]
squares
[1; 4; 9; 16; 25; 36; 49; 64; 81; 100]
let square x = x * x;;
let squares2 = List.map square [1..10]
squares2
[1; 4; 9; 16; 25; 36; 49; 64; 81; 100]
let evenSquares = List.filter (fun x -> x % 2 = 0) squares
evenSqares
[4; 16; 36; 64; 100]
Note the List.map function 'maps' the square function on to the list from 1 to 10, i.e. applies the function to each element. List.filter 'filters' a list by only returning values in the list that pass the predicate function provided. Also note the 'fun x -> f' syntax - this is the F# lambda.
Note that throughout we have not defined any types - the F# compiler/interpreter 'infers' types, i.e. works out what you want from usage. For example:-
let f x = "hi " + x
Here the compiler/interpreter will determine x is a string since you're performing an operation which requires x to be a string. It also determines the return type will be string as well.
When there is ambiguity the compiler makes assumptions, for example:-
let f x y = x + y
Here x and y could be a number of types, but the compiler defaults to int. If you want to define types you can using type annotation:-
let f (x:string) y = x + y
Also note that we have had to enclose x:string in parentheses, we often have to do this to separate parts of a function definition.
Two really useful and heavily used operators in F# are the pipe forward and function composition operators |> and >> respectively.
We define |> thus:-
let (|>) x f = f x
Note that you can define operators in F#, this is pretty cool :-).
This allows you to write things in a clearer way, e.g.:-
[1..10] |> List.map (fun x -> x * x) |> List.filter (fun x -> x % 2 = 0)
Will allow you to obtain the first 10 even squares. That is clearer than:-
List.filter (fun x -> x % 2 = 0) (List.map (fun x -> x * x) [1..10])
Well, at least I think so :-)
Function composition defined by the >> operator is defined as follows:-
let (>>) f g x = g(f(x))
I.e. you forward-pipe an operation only the parameter of the first function remains unspecified. This is useful as you can do the following:-
let mapFilter = List.map (fun x -> x * x) >> List.filter (fun x -> x % 2 = 0)
Here mapFilter will accept a list an input and return the list filtered as before. It's an abbreviated version of:-
let mapFilter = l |> List.map (fun x -> x * x) |> List.filter (fun x -> x % 2 = 0)
If we want to write recursive functions we have to define the function as recursive by placing 'rec' after the let. Examples below.
Some cool stuff:-
Factorial
let rec fact x = if x <= 1 then 1 else x * fact (x-1)
nth Fibonacci Number
let rec fib n = if n <= 1 then n else fib (n-1) + fib (n-2)
FizzBuzz
let (/%) x y = x % y = 0
let fb = function
| x when x /% 15 -> "FizzBuzz"
| x when x /% 3 -> "Fizz"
| x when x /% 5 -> "Buzz"
| x -> x.ToString()
[1..100] |> List.map (fb >> printfn "%s")
Anyway that's a very brief overview, hopefully it helps a little!!

Without doubt, you should purchase Don Syme's excellent book "Expert F#". The book is very well written and is suitable for both beginners and experts alike. In it, you'll find both introductory material and much more challenging material too. At nearly 600 pages it is good value for money.
I found that it taught me a lot of useful techniques for writing more functional C# as well as providing all the reference material I needed to get started writing Windows hosted F# applications.
The book is published by Apress and has an accompanying web site at:
http://www.expert-fsharp.com/default.aspx

#kronoz - well thanks a lot for your long answer, that's a really good place to start from. I'll follow your advices, and look for the book #vecstasy mentioned.
now, let me go coding :-)
let thanksalot = "thanks a lot"
printfn "%s" (thanksalot);;

I've been reading Real World Functional Programming
With examples in F# and C# by:Tomas Petricek
So far I find it very good at teaching F# concepts by showing the implementations in C# on the side. Great for OO Programmers.

The first chapter of my book F# for Scientists is freely available here. We have a series of free F# toy programs here. The first article from our F#.NET Journal is freely available here.

Check out the F# Developer Center. There is also hubFS, a forum dedicated to F#.

If you have the current CTP release in Visual Studio it lets you create a F# Tutorial project, which gives you a Tutorial.fs, exactly containing what it's name suggests.
That tutorial also points to a larger collection of F# examples at Microsoft.
Also, there is an F# samples project going on at CodePlex.
Hope this helps,
Michiel

Related

unexpected return type from list comprehension

I am teaching myself a bit of F# by doing a bit of simple matrix mathematics. I decided to write a set of simple functions for combining two matrices as I thought that this would be a good way of learning list comprehensions. However when I compile it my unit tests produce a type mismatch exception.
//return a column from the matrix as a list
let getColumn(matrix: list<list<double>>, column:int) =
[for row in matrix do yield row.Item(column)]
//return a row from the matrix as a list
let getRow(matrix: list<list<double>>, column:int) =
matrix.Item(column)
//find the minimum width of the matrices in order to avoid index out of range exceptions
let minWidth(matrix1: list<list<double>>,matrix2: list<list<double>>) =
let width1 = [for row in matrix1 do yield row.Length] |> List.min
let width2 = [for row in matrix2 do yield row.Length] |> List.min
if width1 > width2 then width2 else width1
//find the minimum height of the matrices in order to avoid index out of range exceptions
let minHeight(matrix1: list<list<double>>,matrix2: list<list<double>>) =
let height1 = matrix1.Length
let height2 = matrix2.Length
if height1 > height2 then height2 else height1
//combine the two matrices
let concat(matrix1: list<list<double>>,matrix2: list<list<double>>) =
let width = minWidth(matrix1, matrix2)
let height = minHeight(matrix1, matrix2)
[for y in 0 .. height do yield [for x in 0 .. width do yield (List.fold2 (fun acc a b -> acc + (a*b)), getRow(matrix1, y), getColumn(matrix2, x))]]
I was expecting the function to return a list of lists of type
double list list
However what it actually returns looks more like some kind of lambda expression
((int -> int list -> int list -> int) * double list * double list) list list
Can somebody tell me what is being returned, and how to force it to be evaluated into the list of lists that I originally expected?
There's a short answer and a long answer to your question.
The short answer
The short version is that F# functions (like List.fold2) take multiple parameters not with commas the way you think they do, but with spaces in between. I.e., you should NOT call List.fold2 like this:
List.fold2 (function, list1, list2)
but rather like this:
List.fold2 function list1 list2
Now, if you just remove the commas in your List.fold2 call, you'll see that the compiler complains about your getRow(matrix1, y) call, and tells you to put parentheses around them. (And the outer pair of parentheses around List.fold2 isn't actually needed). So this:
(List.fold2 (fun acc a b -> acc + (a*b)), getRow(matrix1, y), getColumn(matrix2, x))
Needs to turn into this:
List.fold2 (fun acc a b -> acc + (a*b)) (getRow(matrix1, y)) (getColumn(matrix2, x))
The long answer
The way F# functions take multiple parameters is actually very different from most other languages such as C#. In fact, all F# functions take exactly one parameter! "But wait," you're probably thinking right now, "you just now showed me the syntax for F# functions taking multiple parameters!" Yes, I did. What's going on under the hood is a combination of currying and partial application. I'd write a long explanation, but Scott Wlaschin has already written one, that's much better than I could have written, so I'll just point you to the https://fsharpforfunandprofit.com/series/thinking-functionally.html series to help you understand what's going on here. (The sections on currying and partial application are the ones you want, but I'd recommend reading the series in order because the later parts build on concepts introduced in earlier parts).
And yes, this "long" answer appears shorter than the "short" answer, but if you go read that series (and then the rest of Scott Wlaschin's excellent site), you'll find that it's much longer than the short answer. :-)
If you have more questions, I'll be happy to try to explain.

F# and lisp-like apply function

For starters, I'm a novice in functional programming and F#, therefore I don't know if it's possible to do such thing at all. So let's say we have this function:
let sum x y z = x + y + z
And for some reason, we want to invoke it using the elements from a list as an arguments. My first attempt was just to do it like this:
//Seq.fold (fun f arg -> f arg) sum [1;2;3]
let rec apply f args =
match args with
| h::hs -> apply (f h) hs
| [] -> f
...which doesn't compile. It seems impossible to determine type of the f with a static type system. There's identical question for Haskell and the only solution uses Data.Dynamic to outfox the type system. I think the closest analog to it in F# is Dynamitey, but I'm not sure if it fits. This code
let dynsum = Dynamitey.Dynamic.Curry(sum, System.Nullable<int>(3))
produces dynsum variable of type obj, and objects of this type cannot be invoked, furthermore sum is not a .NET Delegate.So the question is, how can this be done with/without that library in F#?
F# is a statically typed functional language and so the programming patterns that you use with F# are quite different than those that you'd use in LISP (and actually, they are also different from those you'd use in Haskell). So, working with functions in the way you suggested is not something that you'd do in normal F# programming.
If you had some scenario in mind for this function, then perhaps try asking about the original problem and someone will help you find an idiomatic F# approach!
That said, even though this is not recommended, you can implement the apply function using the powerful .NET reflection capabilities. This is slow and unsafe, but if is occasionally useful.
open Microsoft.FSharp.Reflection
let rec apply (f:obj) (args:obj list) =
let invokeFunc =
f.GetType().GetMethods()
|> Seq.find (fun m ->
m.Name = "Invoke" &&
m.GetParameters().Length = args.Length)
invokeFunc.Invoke(f, Array.ofSeq args)
The code looks at the runtime type of the function, finds Invoke method and calls it.
let sum x y z = x + y + z
let res = apply sum [1;2;3]
let resNum = int res
At the end, you need to convert the result to an int because this is not statically known.

F# Incomplete pattern matches on this expression when using "when"..Why?

I have this simple F# function:
let compareNum x =
let y = 10
match x with
| _ when x = y -> 0
| _ when x > y -> 1
| _ when x < y -> -1
However, F# compiler gives me "Incomplete pattern matches on this expression" warning. In this case, all cases should cover every pattern.
I also see a similar example in "Pattern Matching" section in the 1st edition of Programming F# book by Chris Smith. So something might be changed in the later version of F#?
I think the answer to the previous question (and the comments -- "In general, it is an anti-pattern to have a when guard in the last pattern" -- by kimsk) explain the situation.
However, I would not say that having a guard in the last pattern is an anti-pattern - it is the easiest workaround, but I find this somewhat unfortunate, because the when pattern gives you useful information about the values you can expect - and that makes understanding the program easier. Last time I had this problem, I left it there, at least as a comment:
let compareNum x =
let y = 10
match x with
| _ when x = y -> 0
| _ when x > y -> 1
| _ (*when x < y*) -> -1

What is "monadic reflection"?

What is "monadic reflection"?
How can I use it in F#-program?
Is the meaning of term "reflection" there same as .NET-reflection?
Monadic reflection is essentially a grammar for describing layered monads or monad layering. In Haskell describing also means constructing monads. This is a higher level system so the code looks like functional but the result is monad composition - meaning that without actual monads (which are non-functional) there's nothing real / runnable at the end of the day. Filinski did it originally to try to bring a kind of monad emulation to Scheme but much more to explore theoretical aspects of monads.
Correction from the comment - F# has a Monad equivalent named "Computation Expressions"
Filinski's paper at POPL 2010 - no code but a lot of theory, and of course his original paper from 1994 - Representing Monads. Plus one that has some code: Monad Transformers and Modular Interpreters (1995)
Oh and for people who like code - Filinski's code is on-line. I'll list just one - go one step up and see another 7 and readme. Also just a bit of F# code which claims to be inspired by Filinski
I read through the first Google hit, some slides:
http://www.cs.ioc.ee/mpc-amast06/msfp/filinski-slides.pdf
From this, it looks like
This is not the same as .NET reflection. The name seems to refer to turning data into code (and vice-versa, with reification).
The code uses standard pure-functional operations, so implementation should be easy in F#. (once you understand it)
I have no idea if this would be useful for implementing an immutable cache for a recursive function. It look like you can define mutable operations and convert them to equivalent immutable operations automatically? I don't really understand the slides.
Oleg Kiselyov also has an article, but I didn't even try to read it. There's also a paper from Jonathan Sobel (et al). Hit number 5 is this question, so I stopped looking after that.
As previous answers links describes, Monadic reflection is a concept to bridge call/cc style and Church style programming. To describe these two concepts some more:
F# Computation expressions (=monads) are created with custom Builder type.
Don Syme has a good blog post about this. If I write code to use a builder and use syntax like:
attempt { let! n1 = f inp1
let! n2 = failIfBig inp2
let sum = n1 + n2
return sum }
the syntax is translated to call/cc "call-with-current-continuation" style program:
attempt.Delay(fun () ->
attempt.Bind(f inp1,(fun n1 ->
attempt.Bind(f inp2,(fun n2 ->
attempt.Let(n1 + n2,(fun sum ->
attempt.Return(sum))))))))
The last parameter is the next-command-to-be-executed until the end.
(Scheme-style programming.)
F# is based on OCaml.
F# has partial function application, but it also is strongly typed and has value restriction.
But OCaml don't have value restriction.
OCaml can be used in Church kind of programming, where combinator-functions are used to construct any other functions (or programs):
// S K I combinators:
let I x = x
let K x y = x
let S x y z = x z (y z)
//examples:
let seven = S (K) (K) 7
let doubleI = I I //Won't work in F#
// y-combinator to make recursion
let Y = S (K (S I I)) (S (S (K S) K) (K (S I I)))
Church numerals is a way to represent numbers with pure functions.
let zero f x = x
//same as: let zero = fun f -> fun x -> x
let succ n f x = f (n f x)
let one = succ zero
let two = succ (succ zero)
let add n1 n2 f x = n1 f (n2 f x)
let multiply n1 n2 f = n2(n1(f))
let exp n1 n2 = n2(n1)
Here, zero is a function that takes two functions as parameters: f is applied zero times so this represent the number zero, and x is used to function combination in other calculations (like add). succ function is like plusOne so one = zero |> plusOne.
To execute the functions, the last function will call the other functions with last parameter (x) as null.
(Haskell-style programming.)
In F# value restriction makes this hard. Church numerals can be made with C# 4.0 dynamic keyword (which uses .NET reflection inside). I think there are workarounds to do that also in F#.

How do I define y-combinator without "let rec"?

In almost all examples, a y-combinator in ML-type languages is written like this:
let rec y f x = f (y f) x
let factorial = y (fun f -> function 0 -> 1 | n -> n * f(n - 1))
This works as expected, but it feels like cheating to define the y-combinator using let rec ....
I want to define this combinator without using recursion, using the standard definition:
Y = λf·(λx·f (x x)) (λx·f (x x))
A direct translation is as follows:
let y = fun f -> (fun x -> f (x x)) (fun x -> f (x x));;
However, F# complains that it can't figure out the types:
let y = fun f -> (fun x -> f (x x)) (fun x -> f (x x));;
--------------------------------^
C:\Users\Juliet\AppData\Local\Temp\stdin(6,33): error FS0001: Type mismatch. Expecting a
'a
but given a
'a -> 'b
The resulting type would be infinite when unifying ''a' and ''a -> 'b'
How do I write the y-combinator in F# without using let rec ...?
As the compiler points out, there is no type that can be assigned to x so that the expression (x x) is well-typed (this isn't strictly true; you can explicitly type x as obj->_ - see my last paragraph). You can work around this issue by declaring a recursive type so that a very similar expression will work:
type 'a Rec = Rec of ('a Rec -> 'a)
Now the Y-combinator can be written as:
let y f =
let f' (Rec x as rx) = f (x rx)
f' (Rec f')
Unfortunately, you'll find that this isn't very useful because F# is a strict language,
so any function that you try to define using this combinator will cause a stack overflow.
Instead, you need to use the applicative-order version of the Y-combinator (\f.(\x.f(\y.(x x)y))(\x.f(\y.(x x)y))):
let y f =
let f' (Rec x as rx) = f (fun y -> x rx y)
f' (Rec f')
Another option would be to use explicit laziness to define the normal-order Y-combinator:
type 'a Rec = Rec of ('a Rec -> 'a Lazy)
let y f =
let f' (Rec x as rx) = lazy f (x rx)
(f' (Rec f')).Value
This has the disadvantage that recursive function definitions now need an explicit force of the lazy value (using the Value property):
let factorial = y (fun f -> function | 0 -> 1 | n -> n * (f.Value (n - 1)))
However, it has the advantage that you can define non-function recursive values, just as you could in a lazy language:
let ones = y (fun ones -> LazyList.consf 1 (fun () -> ones.Value))
As a final alternative, you can try to better approximate the untyped lambda calculus by using boxing and downcasting. This would give you (again using the applicative-order version of the Y-combinator):
let y f =
let f' (x:obj -> _) = f (fun y -> x x y)
f' (fun x -> f' (x :?> _))
This has the obvious disadvantage that it will cause unneeded boxing and unboxing, but at least this is entirely internal to the implementation and will never actually lead to failure at runtime.
I would say it's impossible, and asked why, I would handwave and invoke the fact that simply typed lambda calculus has the normalization property. In short, all terms of the simply typed lambda calculus terminate (consequently Y can not be defined in the simply typed lambda calculus).
F#'s type system is not exactly the type system of simply typed lambda calculus, but it's close enough. F# without let rec comes really close to the simply typed lambda calculus -- and, to reiterate, in that language you cannot define a term that does not terminate, and that excludes defining Y too.
In other words, in F#, "let rec" needs to be a language primitive at the very least because even if you were able to define it from the other primitives, you would not be able to type this definition. Having it as a primitive allows you, among other things, to give a special type to that primitive.
EDIT: kvb shows in his answer that type definitions (one of the features absent from the simply typed lambda-calculus but present in let-rec-less F#) allow to get some sort of recursion. Very clever.
Case and let statements in ML derivatives are what makes it Turing Complete, I believe they're based on System F and not simply typed but the point is the same.
System F cannot find a type for the any fixed point combinator, if it could, it wasn't strongly normalizing.
What strongly normalizing means is that any expression has exactly one normal form, where a normal form is an expression that cannot be reduced any further, this differs from untyped where every expression has at max one normal form, it can also have no normal form at all.
If typed lambda calculi could construct a fixed point operator in what ever way, it was quite possible for an expression to have no normal form.
Another famous theorem, the Halting Problem, implies that strongly normalizing languages are not Turing complete, it says that's impossible to decide (different than prove) of a turing complete language what subset of its programs will halt on what input. If a language is strongly normalizing, it's decidable if it halts, namely it always halts. Our algorithm to decide this is the program: true;.
To solve this, ML-derivatives extend System-F with case and let (rec) to overcome this. Functions can thus refer to themselves in their definitions again, making them in effect no lambda calculi at all any more, it's no longer possible to rely on anonymous functions alone for all computable functions. They can thus again enter infinite loops and regain their turing-completeness.
Short answer: You can't.
Long answer:
The simply typed lambda calculus is strongly normalizing. This means it's not Turing equivalent. The reason for this basically boils down to the fact that a Y combinator must either be primitive or defined recursively (as you've found). It simply cannot be expressed in System F (or simpler typed calculi). There's no way around this (it's been proven, after all). The Y combinator you can implement works exactly the way you want, though.
I would suggest you try scheme if you want a real Church-style Y combinator. Use the applicative version given above, as other versions won't work, unless you explicitly add laziness, or use a lazy Scheme interpreter. (Scheme technically isn't completely untyped, but it's dynamically typed, which is good enough for this.)
See this for the proof of strong normalization:
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.127.1794
After thinking some more, I'm pretty sure that adding a primitive Y combinator that behaves exactly the way the letrec defined one does makes System F Turing complete. All you need to do to simulate a Turing machine then is implement the tape as an integer (interpreted in binary) and a shift (to position the head).
Simply define a function taking its own type as a record, like in Swift (there it's a struct) :)
Here, Y (uppercase) is semantically defined as a function that can be called with its own type. In F# terms, it is defined as a record containing a function named call, so for calling a y defined as this type, you have to actually call y.call :)
type Y = { call: Y -> (int -> int) }
let fibonacci n =
let makeF f: int -> int =
fun x ->
if x = 0 then 0 else if x = 1 then 1 else f(x - 1) + f(x - 2)
let y = { call = fun y -> fun x -> (makeF (y.call y)) x }
(y.call y) n
It's not supremely elegant to read but it doesn't resort to recursion for defining a y combinator that is supposed to provide recursion all by itself ^^

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