Is there a standard implementation of F# defaultArg accepting lazy? - f#

I need an implementation of defaultArg function with Lazy as second parameter. Here is a usage example:
defaultArgLazy option (lazy doSomeHeavyWorkThatShouldBeAvoided())
It's quite easy to implement
let defaultArgLazy o (p:'a Lazy) =
match o with
| Some v -> v
| None -> p.Force()
But I wonder if there is a standard implementation that I'm missing.

I don't know of anything built in, but here's a more general version of your function:
module Option =
let lazyDefault f opt =
match opt with
| Some x -> x
| None -> f()
let lazyVal = lazy 1
let opt = None
opt |> Option.lazyDefault lazyVal.Force // returns 1
It takes any function of (unit -> 'a) instead of specifically a Lazy<'a>', so you just pass through Lazy's Force method instead, and retain the guarantee that the evaluation only happens once.
Perhaps Lazy.Force() was added in F# on top of .NET's existing Lazy.Value so that you could use it more idiomatically with higher order functions.
Note that I have swapped the parameter order compared to the built-in defaultArg to be more consistent with functions in other modules and more convenient for piping/currying.

If the lazy value is only used at a later point in code, it might also be an idea to use defaultArg and pass it a Lazy<'a> option as a first argument. That way choosing the option or the default argument is separated from when the result is evaluated.
Taking this idea, the function
let defaultArgLazy opt lazyDef =
(defaultArg (Option.map Lazy.CreateFromValue opt) lazyDef).Value
would do what you want - as does the one by TheQuickBrownFox.

Related

Make Fish in F#

The Kleisli composition operator >=>, also known as the "fish" in Haskell circles, may come in handy in many situations where composition of specialized functions is needed. It works kind of like the >> operator, but instead of composing simple functions 'a -> 'b it confers some special properties on them possibly best expressed as 'a -> m<'b>, where m is either a monad-like type or some property of the function's return value.
Evidence of this practice in the wider F# community can be found e.g. in Scott Wlaschin's Railway oriented programming (part 2) as composition of functions returning the Result<'TSuccess,'TFailure> type.
Reasoning that where there's a bind, there must be also fish, I try to parametrize the canonical Kleisli operator's definition let (>=>) f g a = f a >>= g with the bind function itself:
let mkFish bind f g a = bind g (f a)
This works wonderfully with the caveat that generally one shouldn't unleash special operators on user-facing code. I can compose functions returning options...
module Option =
let (>=>) f = mkFish Option.bind f
let odd i = if i % 2 = 0 then None else Some i
let small i = if abs i > 10 then None else Some i
[0; -1; 9; -99] |> List.choose (odd >=> small)
// val it : int list = [-1; 9]
... or I can devise a function application to the two topmost values of a stack and push the result back without having to reference the data structure I'm operating on explicitly:
module Stack =
let (>=>) f = mkFish (<||) f
type 'a Stack = Stack of 'a list
let pop = function
| Stack[] -> failwith "Empty Stack"
| Stack(x::xs) -> x, Stack xs
let push x (Stack xs) = Stack(x::xs)
let apply2 f =
pop >=> fun x ->
pop >=> fun y ->
push (f x y)
But what bothers me is that the signature val mkFish : bind:('a -> 'b -> 'c) -> f:('d -> 'b) -> g:'a -> a:'d -> 'c makes no sense. Type variables are in confusing order, it's overly general ('a should be a function), and I'm not seeing a natural way to annotate it.
How can I abstract here in the absence of formal functors and monads, not having to define the Kleisli operator explicitly for each type?
You can't do it in a natural way without Higher Kinds.
The signature of fish should be something like:
let (>=>) (f:'T -> #Monad<'U>``) (g:' U -> #Monad<'V>) (x:'T) : #Monad<'V> = bind (f x) g
which is unrepresentable in current .NET type system, but you can replace #Monad with your specific monad, ie: Async and use its corresponding bind function in the implementation.
Having said that, if you really want to use a generic fish operator you can use F#+ which has it already defined by using static constraints. If you look at the 5th code sample here you will see it in action over different types.
Of course you can also define your own, but there is a lot of things to code, in order to make it behave properly in most common scenarios. You can grab the code from the library or if you want I can write a small (but limited) code sample.
The generic fish is defined in this line.
I think in general you really feel the lack of generic functions when using operators, because as you discovered, you need to open and close modules. It's not like functions that you prefix them with the module name, you can do that with operators as well (something like Option.(>=>)) , but then it defeats the whole purpose of using operators, I mean it's no longer an operator.

Can I Access Parameters of a Computation Expression?

Is it possible to create methods or stand-alone functions in a computation expression that can later be used by one of the canonical methods of a computation expression?
I want something like this:
type FormletBuilder(ctx : HttpContext) =
let get_int =
match Int32.TryParse (ctx.Request.["foo"]) with
| (true, n) -> Some n
| _ -> None
//similar definitions for get_date, get_non_empty_str, etc...
member x.Bind (read : 'a option, f : 'a -> option 'a) =
match read with
| Some x -> f(x)
| None -> None
member x.Return (obj) = Some obj
member x.Zero () = None
let person = formlet ctx {
let! id = get_int "id"
let! name = get_non_empty_str "fullname"
return Person(id, name)
}
But the compiler complains that get_int is not defined.
let bindings in class definitions are always private. You can define a member instead.
For an easy solution, you could do:
let formlet = FormletBuilder(ctx)
let person = formlet {
let! id = formlet.get_int "id"
...
}
I understand now that what you actually want is a maybe monad, and the workflow argument is there just to make use of some syntactic sugar? If so, there are a couple other things you can consider doing:
Go Haskell on it all the way and implement a MaybeReader monad, so that both the maybe and the reader parts of it are explicit in type,
Put the sugar away - I understand you don't actually need the context in any core builder members? If so, than maybe it had no business being an argument to the builder in the first place. Have a 'clean' maybe monad, move get_int etc. into a proper module and have them take HttpContext explicitly as an argument.
If you're using F# 3.0 or later, you can define get_int etc. as custom operations of the workflow, which should effectively give you the nice syntax you want to have. Here's a good post about it by Tomas Petricek.
Combine 2. and 3. - instead of a large number of custom operations, have one - ask - which will take an HttpContext -> 'a function and apply ctx to it. Effectively a bastardized version of reader. Then you can move your get_int etc. into a proper module.

what is use cases of F# explicit type parameters?

As I know, explicit type parameters in value definitions is a one way to overcome "value restriction" problem.
Is there another cases when I need to use them?
Upd: I mean "explicitly generic constructs", where type parameter is enclosed in angle brackets, i.e.
let f<'T> x = x
Polymorphic recursion is another case. That is, if you want to use a different generic instantiation within the function body, then you need to use explicit parameters on the definition:
// perfectly balanced tree
type 'a PerfectTree =
| Single of 'a
| Node of ('a*'a) PerfectTree
// need type parameters here
let rec fold<'a,'b> (f:'a -> 'b) (g:'b->'b->'b) : 'a PerfectTree -> 'b = function
| Single a -> f a
| Node t -> t |> fold (fun (a,b) -> g (f a) (f b)) g
let sum = fold id (+)
let ten = sum (Node(Node(Single((1,2),(3,4)))))
This would likely be rare, but when you want to prevent further generalization (ยง14.6.7):
Explicit type parameter definitions on value and member definitions can affect the process of type inference and generalization. In particular, a declaration that includes explicit generic parameters will not be generalized beyond those generic parameters. For example, consider this function:
let f<'T> (x : 'T) y = x
During type inference, this will result in a function of the following type, where '_b is a type inference variable that is yet to be resolved.
f<'T> : 'T -> '_b -> '_b
To permit generalization at these definitions, either remove the explicit generic parameters (if they can be inferred), or use the required number of parameters, as the following example shows:
let throw<'T,'U> (x:'T) (y:'U) = x
Of course, you could also accomplish this with type annotations.
Most obvious example: write a function to calculate the length of a string.
You have to write:
let f (a:string) = a.Length
and you need the annotation. Without the annotation, the compiler can't determine the type of a. Other similar examples exist - particularly when using libraries designed to be used from C#.
Dealing with updated answer:
The same problem applies - string becomes A<string> which has a method get that returns a string
let f (a:A<string>) = a.get().Length

Can the F# type system do this?

I have a lot of various types that have a common purpose, but little else in common. For the sake of explanation, they might as well be along the lines of:
type blah<'a> = Blah of 'a
type huha<'a> = Huha of 'a
I often need to repeat a large chunk of boilerplate that could go inside a function along the lines of:
let f (x:something<int>) (g:something<char> -> float) : float =
But that would require somehow enforcing that the two somethings in the type are the same. In other words, I would like to be able to call the function f as:
f (Blah 1) (fun (b:blah<float>) -> .... )
f (Huha 1) (fun (b:huha<float>) -> .... )
Obviously, a trivial solution is to create a discriminated union of all the types function f could possibly take, and make every g (i.e. f's second argument) check it got whatever type it expected. But that means having a massive type that causes the universe to recompile every time anything changes, and it's not like checking types at runtime helps a great deal either.
So, can someone please see a way of doing what I want in a typesafe way? Many thanks.
As far as I understand, you cannot do this directly in F#.
But maybe operator overloading will help. You would have to implement f for every type, though. But maybe you can delegate to a common implementation.
A code example for operator overloading, ensuring the right types:
type T<'a> =
| T of 'a
static member (.+.) (l:T<int>, r:T<char> -> float) = 42.0
type U<'a> =
| U of 'a
static member (.+.) (l:U<int>, r:U<char> -> float) = 13.1
let ft (t:T<char>) = 42.0
let fu (t:U<char>) = 42.0
let t = T 42 .+. ft
let u = U 13 .+. fu
// does not compile:
let wrong = T 42 .+. fu

Optionally taking the first item in a sequence

I need a function like Seq.head, but returning None instead of throwing an exception when the sequence is empty, i.e., seq<'T> -> 'T option.
There are a jillion ways to do this. Here are several:
let items = Seq.init 10 id
let a = Seq.tryFind (fun _ -> true) items
let b = Seq.tryPick Some items
let c = if Seq.isEmpty items then None else Some (Seq.head items)
let d =
use e = items.GetEnumerator()
if e.MoveNext() then Some e.Current
else None
b is the one I use. Two questions:
Is there a particularly idiomatic way to do this?
Since there's no built-in Seq.tryHead function, does that indicate this shouldn't be necessary, is uncommon, or is better implemented without a function?
UPDATE
tryHead has been added to the standard library in F# 4.0.
I think (b) is probably the most idiomatic, for the same reason #Ramon gave.
I think the lack of Seq.tryHead just means that it is not super common.
I'm not sure, but my guess is that functional languages with Hindley-Milner type inference in general are sparse about implementing such specific functions on collection types because overloading isn't available and composing higher-order functions can be done tersely.
For example, C# Linq extensions are much more exhaustive than functions in F#'s Seq module (which itself is more exhaustive than functions on concrete collection types), and even has IEnumerable.FirstOrDefault. Practically every overload has a variation which performs a map.
I think emphasis on pattern matching and concrete types like list is also a reason.
Now, most of the above is speculation, but I think I may have a notion closer to being objective. I think a lot of the time tryPick and tryFind can be used in the first place instead of filter |> tryHead. For example, I find myself writing code like the following fairly frequently:
open System.Reflection
let ty = typeof<System.String> //suppose this type is actually unknown at compile time
seq {
for name in ["a";"b";"c"] do
yield ty.GetMethod(name)
} |> Seq.tryFind((<>)null)
instead of like
...
seq {
for name in ["a";"b";"c"] do
match ty.GetMethod(name) with
| null -> ()
| mi -> yield mi
} |> tryHead
You could define:
let seqTryHead s = Seq.tryPick Some s
It is of type seq<'a> -> 'a option. Note that I don't beta-reduce because of the generic value limitation.

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