I am trying to define a power function to compute x^y.
let rec powFunA (x,y) =
match (x,y) with
| (_,0) -> 1
| (x,y) -> x * powFunA (x,y-1);;
and
let rec powFunB x y =
match y with
| 0 -> 1
| y -> x * powFunB x y-1;;
The call powFunA (2,5) works and as expected gives me 32 as result. But somehow, I don't understand why, the second call powFunB 2 5 leads to a StackOverflowException.
I also came across a definition:
let rec power = function
| (_,0) -> 1.0 (* 1 *)
| (x,n) -> x * power(x,n-1) (* 2 *)
Can you please explain the absence of parameters and the usage of function on first line of definition.
Thanks.
This stack overflow error has to do with F#'s precedence rules. Consider this expression:
powFunB x y-1
This expression has some function application and the minus operator. In F# (as in all ML languages), function application has the highest precedence ever. Nothing can be more binding.
Therefore, the above expression is understood by the compiler as:
(powFunB x y) - 1
That is, function application powFunB x y first, minus operator second. Now, I hope, it's easy to see why this results in infinite recursion.
To fix, just apply parentheses to override precedence rules:
powFunB x (y-1)
The "parameterless" definition uses F# syntax for defining multicase functions. It's just a shortcut that allows to write = function instead of x = match x with. So, for example, the following two function are equivalent:
let f a = match a with | Some x -> [x] | None -> []
let g = function | Some x -> [x] | None -> []
Just some syntactic sugar, that's all. So the definition you found is exactly equivalent to your first snippet.
Related
I have recently started working on OCAML. I am working from the book Modern Programming Languages, 2nd ed. The first chapter on ML has an exercise requiring the definition of a function max of type int list -> int to return the largest element from a list of integers. There is additionally a hint which suggests the inclusion of a helper function maxhelper which as a second parameter takes the current largest element. Then max is defined as :
fun max x = maxhelper (tl x, hd x)
I am trying to implement this in OCAML. Here is my code :
let max x =
let rec maxhelper x cur_max =
match x with
| [] -> cur_max
| h::t ->
if cur_max < h then maxhelper t h
else maxhelper t cur_max
in maxhelper List.tl(x) List.hd(x)
;;
This results in an error I cannot understand : This expression, i.e. List.tl(x) on the last line has type 'a list -> 'a list
but an expression was expected of type ('b -> 'c -> 'd) list
What puzzles me is when I write the maxhelper function separately and give it arguments [2;3;4] 1 (original list being [1;2;3;4]) it works correctly. Further, if I replace the arguments provided under in as
in maxhelper x 0
The code compiles and works correctly (for non-negative numbers). I am not sure what I have missed regarding passing arguments to in, or the error message I received. Mainly, why does the additional of a List() call cause an error?
You're writing function calls as in an Algolic language, that's the basic problem I think.
Instead of
maxhelper List.tl(x) List.hd(x)
You should have
maxhelper (List.tl x) (List.hd x)
The way you wrote it, there are 4 parameters being passed to maxhelper: List.tl, x, List.hd, x.
I'm new to F#, and functional languages. So this might be stupid question, or duplicated with this Recursive objects in F#?, but I don't know.
Here is a simple Fibonacci function:
let rec fib n =
match n with
| 0 -> 1
| 1 -> 1
| _ -> fib (n - 1) + fib (n - 2)
Its signature is int -> int.
It can be rewritten as:
let rec fib =
fun n ->
match n with
| 0 -> 1
| 1 -> 1
| _ -> fib (n - 1) + fib (n - 2)
Its signature is (int -> int) (in Visual Studio for Mac).
So what's the difference with the previous one?
If I add one more line like this:
let rec fib =
printfn "fib" // <-- this line
fun n ->
match n with
| 0 -> 1
| 1 -> 1
| _ -> fib (n - 1) + fib (n - 2)
The IDE gives me a warning:
warning FS0040: This and other recursive references to the object(s) being defined will be checked for initialization-soundness at runtime through the use of a delayed reference. This is because you are defining one or more recursive objects, rather than recursive functions. This warning may be suppressed by using '#nowarn "40"' or '--nowarn:40'.
How does this line affect the initialization?
What does "recursive object" mean? I can't find it in the documentation.
Update
Thanks for your replies, really nice explanation.
After reading your answers, I have some ideas about the Recursive Object.
First, I made a mistake about the signature. The first two code snippets above have a same signature, int -> int; but the last has signature (int -> int) (note: the signatures have different representation in vscode with Ionide extension).
I think the difference between the two signatures is, the first one means it's just a function, the other one means it's a reference to a function, that is, an object.
And every let rec something with no parameter-list is an object rather than a function, see the function definition, while the second snippet is an exception, possibly optimized by the compiler to a function.
One example:
let rec x = (fun () -> x + 1)() // same warning, says `x` is an recursive object
The only one reason I can think of is the compiler is not smart enough, it throws an warning just because it's a recursive object, like the warning indicates,
This is because you are defining one or more recursive objects, rather than recursive functions
even though this pattern would never have any problem.
let rec fib =
// do something here, if fib invoked here directly, it's definitely an error, not warning.
fun n ->
match n with
| 0 -> 1
| 1 -> 1
| _ -> fib (n - 1) + fib (n - 2)
What do you think about this?
"Recursive objects" are just like recursive functions, except they are, well, objects. Not functions.
A recursive function is a function that references itself, e.g.:
let rec f x = f (x-1) + 1
A recursive object is similar, in that it references itself, except it's not a function, e.g.:
let rec x = x + 1
The above will actually not compile. The F# compiler is able to correctly determine the problem and issue an error: The value 'x' will be evaluated as part of its own definition. Clearly, such definition is nonsensical: in order to calculate x, you need to already know x. Does not compute.
But let's see if we can be more clever. How about if I close x in a lambda expression?
let rec x = (fun() -> x + 1) ()
Here, I wrap the x in a function, and immediately call that function. This compiles, but with a warning - the same warning that you're getting, something about "checking for initialization-soundness at runtime".
So let's go to runtime:
> let rec x = (fun() -> x + 1) ()
System.InvalidOperationException: ValueFactory attempted to access the Value property of this instance.
Not surprisingly, we get an error: turns out, in this definition, you still need to know x in order to calculate x - same as with let rec x = x + 1.
But if this is the case, why does it compile at all? Well, it just so happens that, in general, it is impossible to strictly prove that x will or will not access itself during initialization. The compiler is just smart enough to notice that it might happen (and this is why it issues the warning), but not smart enough to prove that it will definitely happen.
So in cases like this, in addition to issuing a warning, the compiler will install a runtime guard, which will check whether x has already been initialized when it's being accessed. The compiled code with such guard might look something like this:
let mutable x_initialized = false
let rec x =
let x_temp =
(fun() ->
if not x_initialized then failwith "Not good!"
else x + 1
) ()
x_initialized <- true
x_temp
(the actual compiled code looks differently of course; use ILSpy to look if you're curious)
In certain special cases, the compiler can prove one way or another. In other cases it can't, so it installs runtime protection:
// Definitely bad => compile-time error
let rec x = x + 1
// Definitely good => no errors, no warnings
let rec x = fun() -> x() + 1
// Might be bad => compile-time warning + runtime guard
let rec x = (fun() -> x+1) ()
// Also might be bad: no way to tell what the `printfn` call will do
let rec x =
printfn "a"
fun() -> x() + 1
There's a major difference between the last two versions. Notice adding a printfn call to the first version generates no warning, and "fib" will be printed each time the function recurses:
let rec fib n =
printfn "fib"
match n with
| 0 -> 1
| 1 -> 1
| _ -> fib (n - 1) + fib (n - 2)
> fib 10;;
fib
fib
fib
...
val it : int = 89
The printfn call is part of the recursive function's body. But the 3rd/final version only prints "fib" once when the function is defined then never again.
What's the difference? In the 3rd version you're not defining just a recursive function, because there are other expressions creating a closure over the lambda, resulting in a recursive object. Consider this version:
let rec fib3 =
let x = 1
let y = 2
fun n ->
match n with
| 0 -> x
| 1 -> x
| _ -> fib3 (n - x) + fib3 (n - y)
fib3 is not a plain recursive function; there's a closure over the function capturing x and y (and same for the printfn version, although it's just a side-effect). This closure is the "recursive object" referred to in the warning. x and y will not be redefined in each recursion; they're part of the root-level closure/recursive object.
From the linked question/answer:
because [the compiler] cannot guarantee that the reference won't be accessed before it is initialized
Although it doesn't apply in your particular example, it's impossible for the compiler to know whether you're doing something harmless, or potentially referencing/invoking the lambda in fib3 definition before fib3 has a value/has been initialized. Here's another good answer explaining the same.
Idiomatic F# can nicely represent the classic recursive expression data structure:
type Expression =
| Number of int
| Add of Expression * Expression
| Multiply of Expression * Expression
| Variable of string
together with recursive functions thereon:
let rec simplify_add (exp: Expression): Expression =
match exp with
| Add (x, Number 0) -> x
| Add (Number 0, x) -> x
| _ -> exp
... oops, that doesn't work as written; simplify_add needs to recur into subexpressions. In this toy example that's easy enough to do, only a couple of extra lines of code, but in a real program there would be dozens of expression types; one would prefer to avoid adding dozens of lines of boilerplate to every function that operates on expressions.
Is there any way to express 'by default, recur on subexpressions'? Something like:
let rec simplify_add (exp: Expression): Expression =
match exp with
| Add (x, Number 0) -> x
| Add (Number 0, x) -> x
| _ -> recur simplify_add exp
where recur might perhaps be some sort of higher-order function that uses reflection to look up the type definition or somesuch?
Unfortunately, F# does not give you any recursive function for processing your data type "for free". You could probably generate one using reflection - this would be valid if you have a lot of recursive types, but it might not be worth it in normal situations.
There are various patterns that you can use to hide the repetition though. One that I find particularly nice is based on the ExprShape module from standard F# libraries. The idea is to define an active pattern that gives you a view of your type as either leaf (with no nested sub-expressions) or node (with a list of sub-expressions):
type ShapeInfo = Shape of Expression
// View expression as a node or leaf. The 'Shape' just stores
// the original expression to keep its original structure
let (|Leaf|Node|) e =
match e with
| Number n -> Leaf(Shape e)
| Add(e1, e2) -> Node(Shape e, [e1; e2])
| Multiply(e1, e2) -> Node(Shape e, [e1; e2])
| Variable s -> Leaf(Shape e)
// Reconstruct an expression from shape, using new list
// of sub-expressions in the node case.
let FromLeaf(Shape e) = e
let FromNode(Shape e, args) =
match e, args with
| Add(_, _), [e1; e2] -> Add(e1, e2)
| Multiply(_, _), [e1; e2] -> Multiply(e1, e2)
| _ -> failwith "Wrong format"
This is some boilerplate code that you'd have to write. But the nice thing is that we can now write the recursive simplifyAdd function using just your special cases and two additional patterns for leaf and node:
let rec simplifyAdd exp =
match exp with
// Special cases for this particular function
| Add (x, Number 0) -> x
| Add (Number 0, x) -> x
// This now captures all other recursive/leaf cases
| Node (n, exps) -> FromNode(n, List.map simplifyAdd exps)
| Leaf _ -> exp
Is it possible to write recursive anonymous functions in SML? I know I could just use the fun syntax, but I'm curious.
I have written, as an example of what I want:
val fact =
fn n => case n of
0 => 1
| x => x * fact (n - 1)
The anonymous function aren't really anonymous anymore when you bind it to a
variable. And since val rec is just the derived form of fun with no
difference other than appearance, you could just as well have written it using
the fun syntax. Also you can do pattern matching in fn expressions as well
as in case, as cases are derived from fn.
So in all its simpleness you could have written your function as
val rec fact = fn 0 => 1
| x => x * fact (x - 1)
but this is the exact same as the below more readable (in my oppinion)
fun fact 0 = 1
| fact x = x * fact (x - 1)
As far as I think, there is only one reason to use write your code using the
long val rec, and that is because you can easier annotate your code with
comments and forced types. For examples if you have seen Haskell code before and
like the way they type annotate their functions, you could write it something
like this
val rec fact : int -> int =
fn 0 => 1
| x => x * fact (x - 1)
As templatetypedef mentioned, it is possible to do it using a fixed-point
combinator. Such a combinator might look like
fun Y f =
let
exception BlackHole
val r = ref (fn _ => raise BlackHole)
fun a x = !r x
fun ta f = (r := f ; f)
in
ta (f a)
end
And you could then calculate fact 5 with the below code, which uses anonymous
functions to express the faculty function and then binds the result of the
computation to res.
val res =
Y (fn fact =>
fn 0 => 1
| n => n * fact (n - 1)
)
5
The fixed-point code and example computation are courtesy of Morten Brøns-Pedersen.
Updated response to George Kangas' answer:
In languages I know, a recursive function will always get bound to a
name. The convenient and conventional way is provided by keywords like
"define", or "let", or "letrec",...
Trivially true by definition. If the function (recursive or not) wasn't bound to a name it would be anonymous.
The unconventional, more anonymous looking, way is by lambda binding.
I don't see what unconventional there is about anonymous functions, they are used all the time in SML, infact in any functional language. Its even starting to show up in more and more imperative languages as well.
Jesper Reenberg's answer shows lambda binding; the "anonymous"
function gets bound to the names "f" and "fact" by lambdas (called
"fn" in SML).
The anonymous function is in fact anonymous (not "anonymous" -- no quotes), and yes of course it will get bound in the scope of what ever function it is passed onto as an argument. In any other cases the language would be totally useless. The exact same thing happens when calling map (fn x => x) [.....], in this case the anonymous identity function, is still in fact anonymous.
The "normal" definition of an anonymous function (at least according to wikipedia), saying that it must not be bound to an identifier, is a bit weak and ought to include the implicit statement "in the current environment".
This is in fact true for my example, as seen by running it in mlton with the -show-basis argument on an file containing only fun Y ... and the val res ..
val Y: (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b
val res: int32
From this it is seen that none of the anonymous functions are bound in the environment.
A shorter "lambdanonymous" alternative, which requires OCaml launched
by "ocaml -rectypes":
(fun f n -> f f n)
(fun f n -> if n = 0 then 1 else n * (f f (n - 1))
7;; Which produces 7! = 5040.
It seems that you have completely misunderstood the idea of the original question:
Is it possible to write recursive anonymous functions in SML?
And the simple answer is yes. The complex answer is (among others?) an example of this done using a fix point combinator, not a "lambdanonymous" (what ever that is supposed to mean) example done in another language using features not even remotely possible in SML.
All you have to do is put rec after val, as in
val rec fact =
fn n => case n of
0 => 1
| x => x * fact (n - 1)
Wikipedia describes this near the top of the first section.
let fun fact 0 = 1
| fact x = x * fact (x - 1)
in
fact
end
This is a recursive anonymous function. The name 'fact' is only used internally.
Some languages (such as Coq) use 'fix' as the primitive for recursive functions, while some languages (such as SML) use recursive-let as the primitive. These two primitives can encode each other:
fix f => e
:= let rec f = e in f end
let rec f = e ... in ... end
:= let f = fix f => e ... in ... end
In languages I know, a recursive function will always get bound to a name. The convenient and conventional way is provided by keywords like "define", or "let", or "letrec",...
The unconventional, more anonymous looking, way is by lambda binding. Jesper Reenberg's answer shows lambda binding; the "anonymous" function gets bound to the names "f" and "fact" by lambdas (called "fn" in SML).
A shorter "lambdanonymous" alternative, which requires OCaml launched by "ocaml -rectypes":
(fun f n -> f f n)
(fun f n -> if n = 0 then 1 else n * (f f (n - 1))
7;;
Which produces 7! = 5040.
I feel like this should be fairly obvious, or easy, but I just can't get it. What I want to do is apply a function to a list (using map) but only if a condition is held. Imagine you only wanted to divide the numbers which were even:
map (`div` 2) (even) [1,2,3,4]
And that would give out [1,1,3,2] since only the even numbers would have the function applied to them. Obviously this doesn't work, but is there a way to make this work without having to write a separate function that you can give to map? filter is almost there, except I also want to keep the elements which the condition doesn't hold for, and just not apply the function to them.
If you don't want to define separate function, then use lambda.
map (\x -> if (even x) then (x `div` 2) else x) [1,2,3,4]
Or instead of a map, list comprehension, bit more readable I think.
[if (even x) then (x `div` 2) else x | x <- [1,2,3,4]]
mapIf p f = map (\x -> if p x then f x else x)
In addition to the answer of PiotrLegnica: Often, it's easier to read if you declare a helper function instead of using a lambda. Consider this:
map helper [1..4] where
helper x | even x = x `div` 2
| otherwise = x
([1..4] is sugar for [1,2,3,4])
If you want to remove all the other elements instead, consider using filter. filter removes all elements that don't satisfy the predicate:
filter even [1..4] -> [2,4]
So you can build a pipe of mapand filter than or use list-comprehension instead:
map (`div` 2) $ filter even [1..4]
[x `div` 2 | x <- [1..4], even x]
Choose whatever you like best.
Make your own helper function maker:
ifP pred f x =
if pred x then f x
else x
custom_f = ifP even f
map custom_f [..]
(caveat: I don't have access to a compiler right now. I guess this works OK...)
I like the other, more general solutions, but in your very special case you can get away with
map (\x -> x `div` (2 - x `mod` 2)) [1..4]
Mostly a rip off of existing answers, but according to my biased definition of "readable" (I like guards more than ifs, and where more than let):
mapIf p f = map f'
where f' x | p x = f x | otherwise = x
ghci says it probably works
ghci> let mapIf p f = map f' where f' x | p x = f x | otherwise = x
ghci> mapIf even (+1) [1..10]
[1,3,3,5,5,7,7,9,9,11]