I have an extension method
type System.Int32 with
member this.Thousand() = this * 1000
but it requires me to write like this
(5).Thousand()
I'd love to get rid of both parenthesis, starting with making it a property instead of a method (for learning sake) how do I make this a property?
Jon's answer is one way to do it, but for a read-only property there's also a more concise way to write it:
type System.Int32 with
member this.Thousand = this * 1000
Also, depending on your preferences, you may find it more pleasing to write 5 .Thousand (note the extra space) than (5).Thousand (but you won't be able to do just 5.Thousand, or even 5.ToString()).
I don't really know F# (shameful!) but based on this blog post, I'd expect:
type System.Int32 with
member this.Thousand
with get() = this * 1000
I suspect that won't free you from the first set of parentheses (otherwise F# may try to parse the whole thing as a literal), but it should help you with the second.
Personally I wouldn't use this sort of thing for a "production" extension, but it's useful for test code where you're working with a lot of values.
In particular, I've found it neat to have extension methods around dates, e.g. 19.June(1976) as a really simple, easy-to-read way of building up test data. But not for production code :)
It's not beautiful, but if you really want a function that will work for any numeric type, you can do this:
let inline thousand n =
let one = LanguagePrimitives.GenericOne
let thousand =
let rec loop n i =
if i < 1000 then loop (n + one) (i + 1)
else n
loop one 1
n * thousand
5.0 |> thousand
5 |> thousand
5I |> thousand
Related
I'd like the example computation expression and values below to return 6. For some the numbers aren't yielding like I'd expect. What's the step I'm missing to get my result? Thanks!
type AddBuilder() =
let mutable x = 0
member _.Yield i = x <- x + i
member _.Zero() = 0
member _.Return() = x
let add = AddBuilder()
(* Compiler tells me that each of the numbers in add don't do anything
and suggests putting '|> ignore' in front of each *)
let result = add { 1; 2; 3 }
(* Currently the result is 0 *)
printfn "%i should be 6" result
Note: This is just for creating my own computation expression to expand my learning. Seq.sum would be a better approach. I'm open to the idea that this example completely misses the value of computation expressions and is no good for learning.
There is a lot wrong here.
First, let's start with mere mechanics.
In order for the Yield method to be called, the code inside the curly braces must use the yield keyword:
let result = add { yield 1; yield 2; yield 3 }
But now the compiler will complain that you also need a Combine method. See, the semantics of yield is that each of them produces a finished computation, a resulting value. And therefore, if you want to have more than one, you need some way to "glue" them together. This is what the Combine method does.
Since your computation builder doesn't actually produce any results, but instead mutates its internal variable, the ultimate result of the computation should be the value of that internal variable. So that's what Combine needs to return:
member _.Combine(a, b) = x
But now the compiler complains again: you need a Delay method. Delay is not strictly necessary, but it's required in order to mitigate performance pitfalls. When the computation consists of many "parts" (like in the case of multiple yields), it's often the case that some of them should be discarded. In these situation, it would be inefficient to evaluate all of them and then discard some. So the compiler inserts a call to Delay: it receives a function, which, when called, would evaluate a "part" of the computation, and Delay has the opportunity to put this function in some sort of deferred container, so that later Combine can decide which of those containers to discard and which to evaluate.
In your case, however, since the result of the computation doesn't matter (remember: you're not returning any results, you're just mutating the internal variable), Delay can just execute the function it receives to have it produce the side effects (which are - mutating the variable):
member _.Delay(f) = f ()
And now the computation finally compiles, and behold: its result is 6. This result comes from whatever Combine is returning. Try modifying it like this:
member _.Combine(a, b) = "foo"
Now suddenly the result of your computation becomes "foo".
And now, let's move on to semantics.
The above modifications will let your program compile and even produce expected result. However, I think you misunderstood the whole idea of the computation expressions in the first place.
The builder isn't supposed to have any internal state. Instead, its methods are supposed to manipulate complex values of some sort, some methods creating new values, some modifying existing ones. For example, the seq builder1 manipulates sequences. That's the type of values it handles. Different methods create new sequences (Yield) or transform them in some way (e.g. Combine), and the ultimate result is also a sequence.
In your case, it looks like the values that your builder needs to manipulate are numbers. And the ultimate result would also be a number.
So let's look at the methods' semantics.
The Yield method is supposed to create one of those values that you're manipulating. Since your values are numbers, that's what Yield should return:
member _.Yield x = x
The Combine method, as explained above, is supposed to combine two of such values that got created by different parts of the expression. In your case, since you want the ultimate result to be a sum, that's what Combine should do:
member _.Combine(a, b) = a + b
Finally, the Delay method should just execute the provided function. In your case, since your values are numbers, it doesn't make sense to discard any of them:
member _.Delay(f) = f()
And that's it! With these three methods, you can add numbers:
type AddBuilder() =
member _.Yield x = x
member _.Combine(a, b) = a + b
member _.Delay(f) = f ()
let add = AddBuilder()
let result = add { yield 1; yield 2; yield 3 }
I think numbers are not a very good example for learning about computation expressions, because numbers lack the inner structure that computation expressions are supposed to handle. Try instead creating a maybe builder to manipulate Option<'a> values.
Added bonus - there are already implementations you can find online and use for reference.
1 seq is not actually a computation expression. It predates computation expressions and is treated in a special way by the compiler. But good enough for examples and comparisons.
I have an object that can be neatly described by a discriminated union. The tree that it represents has some properties that can be easily updated when the tree is modified (but remaining immutable) but that are relatively expensive to recalculate.
I would like to store those properties along with the object as cached values but I don't want to put them into each of the discriminated union cases so I figured a member variable would fit here.
The question is then, how do I change the member value (when I modify the tree) without mutating the actual object? I know I could modify the tree and then mutate that copy without ruining purity but that seems like a wrong way to go about it to me. It would make sense to me if there was some predefined way to change a property but so that the result of the operation is a new object with that property changed.
To clarify, when I say modify I mean doing it in a functional way. Like (::) "appends" to the beginning of a list. I'm not sure what the correct terminology is here.
F# actually has syntax for copy and update records.
The syntax looks like this:
let myRecord3 = { myRecord2 with Y = 100; Z = 2 }
(example from the MSDN records page - http://msdn.microsoft.com/en-us/library/dd233184.aspx).
This allows the record type to be immutable, and for large parts of it to be preserved, whilst only a small part is updated.
The cleanest way to go about it would really be to carry the 'cached' value attached to the DU (as part of the case) in one way or another. I could think of several ways to implement this, I'll just give you one, where there are separate cases for the cached and non-cached modes:
type Fraction =
| Frac of int * int
| CachedFrac of (int * int) * decimal
member this.AsFrac =
match this with
| Frac _ -> this
| CachedFrac (tup, _) -> Frac tup
An entirely different option would be to keep the cached values in a separate dictionary, this is something that makes sense if all you want to do is save some time recalculating them.
module FracCache =
let cache = System.Collections.Generic.Dictionary<Fraction, decimal>()
let modify (oldFrac: Fraction) (newFrac: Fraction) =
cache.[newFrac] <- cache.[oldFrac] + 1 // need to check if oldFrac has a cached value as well.
Basically what memoize would give you plus you have more control over it.
Given the following two approaches, what would the cons and pros of both, when it comes to function composition?
Approach 1
let isNameTaken source name =
source |> Query.Exists(fun z -> z.Name = name)
let usage : Customer = isNameTaken source "Test"
Approach 2
let isNameTaken f name =
f(fun z -> z.Name = name)
let usage : Customer = isNameTaken (source |> Query.Exists) "Test"
Is it just silly to pass (source |> Query.Exists) in Approach 2 - is it too extreme?
It depends on the wider context. I would generally prefer the first approach, unless you have some really good reason for using the second style (e.g. there is a number of functions similar to Query.Exists that you need to apply in a similar style).
Aside - I think your second example has a couple of issues (e.g. the piping in source |> Query.Exists would have to be replaced with (fun pred -> source |> Query.Exists pred) which makes it uglier.
Even then, the second approach does not really give you much benefit - your isNameTaken is simply a function that tests whether a customer name equals a given name and then it passes that as an argument to some f - you could just define a function that tests name equality and write something like this:
let nameEquals name (customer:Customer) =
customer.Name = name
let usage = source |> Query.Exists (nameEquals "Test")
More generally, I think it is always preferable to write code so that the caller can compose the pieces that are available to them (like Query.Exists, nameEquals etc.) rather than In a way that requires the caller to fill some holes of a particular required shape (e.g. implement a function with specified signature).
I think the answer to your question has to do with two main criteria. Which is more important, the readability of the code or the decoupling of the query from isNameTaken. In this particular case, I'm not sure that you get much at all from decoupling the query and it also seems like your decoupling is partial.
The thing I don't like about this is that in both cases, you've got z.Name tightly coupled into isNameTaken, which means that isNameTaken needs to know about the type of z. If that's OK with you then fine.
Extremely just-started-yesterday new to F#.
What I want: To write code that parses the string "2 + 2" into (using as an example code from the tutorial project) Expr.Add(Expr.Num 2, Expr.Num 2) for evaluation. Some help to at least point me in the right direction or tell me it's too complex for my first F# project. (This is how I learn things: By bashing my head against stuff that's hard)
What I have: My best guess at code to extract the numbers. Probably horribly off base. Also, a lack of clue.
let script = "2 + 2";
let rec scriptParse xs =
match xs with
| [] -> (double)0
| y::ys -> (double)y
let split = (script.Split([|' '|]))
let f x = (split[x]) // "This code is not a function and cannot be applied."
let list = [ for x in 0..script.Length -> f x ]
let result = scriptParse
Thanks.
The immediate issue that you're running into is that split is an array of strings. To access an element of this array, the syntax is split.[x], not split[x] (which would apply split to the singleton list [x], assuming it were a function).
Here are a few other issues:
Your definition of list is probably wrong: x ranges up to the length of script, not the length of the array split. If you want to convert an array or other sequence to a list you can just use List.ofSeq or Seq.toList instead of an explicit list comprehension [...].
Your "casts" to double are a bit odd - that's not the right syntax for performing conversions in F#, although it will work in this case. double is a function, so the parentheses are unnecessary and what you are doing is really calling double 0 and double y. You should just use 0.0 for the first case, and in the second case, it's unclear what you are converting from.
In general, it would probably be better to do a bit more design up front to decide what your overall strategy will be, since it's not clear to me that you'll be able to piece together a working parser based on your current approach. There are several well known techniques for writing a parser - are you trying to use a particular approach?
This is a pretty simple question, and I just wanted to check that what I'm doing and how I'm interpreting the F# makes sense. If I have the statement
let printRandom =
x = MyApplication.getRandom()
printfn "%d" x
x
Instead of creating printRandom as a function, F# runs it once and then assigns it a value. So, now, when I call printRandom, instead of getting a new random value and printing it, I simply get whatever was returned the first time. I can get around this my defining it as such:
let printRandom() =
x = MyApplication.getRandom()
printfn "%d" x
x
Is this the proper way to draw this distinction between parameter-less functions and values? This seems less than ideal to me. Does it have consequences in currying, composition, etc?
The right way to look at this is that F# has no such thing as parameter-less functions. All functions have to take a parameter, but sometimes you don't care what it is, so you use () (the singleton value of type unit). You could also make a function like this:
let printRandom unused =
x = MyApplication.getRandom()
printfn "%d" x
x
or this:
let printRandom _ =
x = MyApplication.getRandom()
printfn "%d" x
x
But () is the default way to express that you don't use the parameter. It expresses that fact to the caller, because the type is unit -> int not 'a -> int; as well as to the reader, because the call site is printRandom () not printRandom "unused".
Currying and composition do in fact rely on the fact that all functions take one parameter and return one value.
The most common way to write calls with unit, by the way, is with a space, especially in the non .NET relatives of F# like Caml, SML and Haskell. That's because () is a singleton value, not a syntactic thing like it is in C#.
Your analysis is correct.
The first instance defines a value and not a function. I admit this caught me a few times when I started with F# as well. Coming from C# it seems very natural that an assignment expression which contains multiple statements must be a lambda and hence delay evaluated.
This is just not the case in F#. Statements can be almost arbitrarily nested (and it rocks for having locally scoped functions and values). Once you get comfortable with this you start to see it as an advantage as you can create functions and continuations which are inaccessible to the rest of the function.
The second approach is the standard way for creating a function which logically takes no arguments. I don't know the precise terminology the F# team would use for this declaration though (perhaps a function taking a single argument of type unit). So I can't really comment on how it would affect currying.
Is this the proper way to draw this
distinction between parameter-less
functions and values? This seems less
than ideal to me. Does it have
consequences in currying, composition,
etc?
Yes, what you describe is correct.
For what its worth, it has a very interesting consequence able to partially evaluate functions on declaration. Compare these two functions:
// val contains : string -> bool
let contains =
let people = set ["Juliet"; "Joe"; "Bob"; "Jack"]
fun person -> people.Contains(person)
// val contains2 : string -> bool
let contains2 person =
let people = set ["Juliet"; "Joe"; "Bob"; "Jack"]
people.Contains(person)
Both functions produce identical results, contains creates its people set on declaration and reuses it, whereas contains2 creates its people set everytime you call the function. End result: contains is slightly faster. So knowing the distinction here can help you write faster code.
Assignment bodies looking like function bodies have cought a few programmers unaware. You can make things even more interesting by having the assignment return a function:
let foo =
printfn "This runs at startup"
(fun () -> printfn "This runs every time you call foo ()")
I just wrote a blog post about it at http://blog.wezeku.com/2010/08/23/values-functions-and-a-bit-of-both/.