Am I being stupid?
Some things look nicer in a monad/computation expression, I have lots of code using the seq computation expression, but everytime I hit an option<> I have to revert to "Option.map" etc.
Slightly jarring (when I was doing this in C# I wrote a linq operator for a IMaybe<> type and it all looked nice and consistent).
I can, but don't especially want to write one, there must be one (or more) out there, which one do people use?
i.e.
so rather than
let postCodeMaybe =
personMaybe
|> Option.bind (fun p -> p.AddressMaybe())
|> Option.map (fun -> p.Postcode())
we can go
let postCodeMaybe =
option {
for p in personMaybe do
for a in p.AddressMaybe() do
a.Postcode()
}
I have no problem with the former code, except that in my context it exists in lots of "seq" computational expressions that look like the latter (and some developers who will look at this code will come from a C#/LINQ background which are basically the latter).
There is no option CE in FSharp.Core but it exists in some libraries.
I think one of the reasons why many CEs are not provided in FSharp.Core is because there are many different ways of doing them, if you google for an option computation expression builder you'll find some are strict, others lazy, some support side-effects others support blending multiple return values into one.
Regarding libraries, there is one in F#x called maybe then you have F#+ which provides generic computation expressions so you have something like Linq where as long as the type implement some methods the CE is ready to use, for options you can use let x:option<_> = monad' { ... or let x:option<_> = monad.plus' { ... if you prefer to have the one that allows multiple return values.
Your code will look like:
let x: option<_> = option {
let! p = personMaybe
let! a = p.AddressMaybe()
return a }
Where option is your option builder, could be one of the above mentioned.
We use FsToolkit.ErrorHandling. It is simple, actively maintained and works well.
It has CEs for Option, Result, ResultOption, Validation, AsyncResult, AsyncResultOption.
https://www.nuget.org/packages/FsToolkit.ErrorHandling/
https://github.com/demystifyfp/FsToolkit.ErrorHandling
Related
I seem to often run into cases where I want to generate some complex structure, but a special variation with a member type generated differently.
For example, consider this tree
type Tree<'LeafData,'INodeData> =
| LeafNode of 'LeafData
| InternalNode of 'INodeData * Tree<'LeafData,'INodeData> list
I want to generate cases like
No internal node is childless
There are no leaf-type nodes
Only a limited subset of leaf types are used
These are simple to do if I override all generation of a corresponding child type.
The problem is that it seems register is inherently a thread-level action, and there is no gen-local alternative.
For example, what I want could look like
let limitedLeafs =
gen {
let leafGen = Arb.generate<LeafType> |> Gen.filter isAllowedLeaf
do! registerContextualArb (leafGen |> Arb.fromGen)
return! Arb.generate<Tree<NodeType, LeafType>>
}
This Tree example specifically can work around with some creative type shuffling, but that's not always possible.
It's also possible to use some sort of recursive map that enforces assumptions, but that seems relatively complex if the above is possible. I might be misunderstanding the nature of FsCheck generators though.
Does anyone know how to accomplish this kind of gen-local override?
There's a few options here - I'm assuming you're on FsCheck 2.x but keep scrolling for an option in FsCheck 3.
The first is the most natural one but is more work, which is to break down the generator explicitly to the level you need, and then put humpty dumpty together again. I.e don't rely on the type-based generator derivation so much - if I understand your example correctly that would mean implementing a recursive generator - relying on Arb.generate<LeafType> for the generic types.
Second option - Config has an Arbitrary field which you can use to override Arbitrary instances. These overrides will take effect even if the overridden types are part of the automatically generated ones. So as a sketch you could try:
Check.One ({Config.Quick with Arbitrary = [| typeof<MyLeafArbitrary>) |]) (fun safeTree -> ...)
More extensive example which uses FsCheck.Xunit's PropertyAttribute but the principle is the same, set on the Config instead.
Final option! :) In FsCheck 3 (prerelease) you can configure this via a new (as of yet undocumented) concept ArbMap which makes the map from type to Arbitrary instance explicit, instead of this static global nonsense in 2.x (my bad of course. seemed like a good idea at the time.) The implementation is here which may not tell you all that much - the idea is that you put an ArbMap instance together which contains your "safe" generators for the subparts, then you ArbMap.mergeWith that safe map with ArbMap.defaults (thus overriding the default generators with your safe ones, in the resulting ArbMap) and then you use ArbMap.arbitrary or ArbMap.generate with the resulting map.
Sorry for the long winded explanation - but all in all that should give you the best of both worlds - you can reuse the generic union type generator in FsCheck, while surgically overriding certain types in that context.
FsCheck guidance on this is:
To define a generator that generates a subset of the normal range of values for an existing type, say all the even ints, it makes properties more readable if you define a single-case union case, and register a generator for the new type:
As an example, they suggest you could define arbitrary even integers like this:
type EvenInt = EvenInt of int with
static member op_Explicit(EvenInt i) = i
type ArbitraryModifiers =
static member EvenInt() =
Arb.from<int>
|> Arb.filter (fun i -> i % 2 = 0)
|> Arb.convert EvenInt int
Arb.register<ArbitraryModifiers>() |> ignore
You could then generate and test trees whose leaves are even integers like this:
let ``leaves are even`` (tree : Tree<EvenInt, string>) =
let rec leaves = function
| LeafNode leaf -> [leaf]
| InternalNode (_, children) ->
children |> List.collect leaves
leaves tree
|> Seq.forall (fun (EvenInt leaf) ->
leaf % 2 = 0)
Check.Quick ``leaves are even`` // output: Ok, passed 100 tests.
To be honest, I like your idea of a "gen-local override" better, but I don't think FsCheck supports it.
While working through Expert F# again, I decided to implement the application for manipulating algebraic expressions. This went well and now I've decided as a next exercise to expand on that by building a more advanced application.
My first idea was to have a setup that allows for a more extendible way of creating functions without having to recompile. To that end I have something like:
type IFunction =
member x.Name : string with get
/// additional members omitted
type Expr =
| Num of decimal
| Var of string
///... omitting some types here that don't matter
| FunctionApplication of IFunction * Expr list
So that say a Sin(x) could be represented a:
let sin = { new IFunction() with member x.Name = "SIN" }
let sinExpr = FunctionApplication(sin,Var("x"))
So far all good, but the next idea that I would like to implement is having additional interfaces to represent function of properties. E.g.
type IDifferentiable =
member Derivative : int -> IFunction // Get the derivative w.r.t a variable index
One of the ideas the things I'm trying to achieve here is that I implement some functions and all the logic for them and then move on to the next part of the logic I would like to implement. However, as it currently stands, that means that with every interface I add, I have to revisit all the IFunctions that I've implemented. Instead, I'd rather have a function:
let makeDifferentiable (f : IFunction) (deriv : int -> IFunction) =
{ f with
interface IDifferentiable with
member x.Derivative = deriv }
but as discussed in this question, that is not possible. The alternative that is possible, doesn't meet my extensibility requirement. My question is what alternatives would work well?
[EDIT] I was asked to expand on the "doesn't meet my extenibility requirement" comment. The way this function would work is by doing something like:
let makeDifferentiable (deriv : int -> IFunction) (f : IFunction)=
{ new IFunction with
member x.Name = f.Name
interface IDifferentiable with
member x.Derivative = deriv }
However, ideally I would keep on adding additional interfaces to an object as I add them. So if I now wanted to add an interface that tell whether on function is even:
type IsEven =
abstract member IsEven : bool with get
then I would like to be able to (but not obliged, as in, if I don't make this change everything should still compile) to change my definition of a sine from
let sin = { new IFunction with ... } >> (makeDifferentiable ...)
to
let sin = { new IFunction with ... } >> (makeDifferentiable ...) >> (makeEven false)
The result of which would be that I could create an object that implements the IFunction interface as well as potentially, but not necessarily a lot of different other interfaces as well; the operations I'd then define on them, would potentially be able to optimize what they are doing based on whether or not a certain function implements an interface. This will also allow me to add additional features/interfaces/operations first without having to change the functions I've defined (though they wouldn't take advantage of the additional features, things wouldn't be broken either.[/EDIT]
The only thing I can think of right now is to create a dictionary for each feature that I'd like to implement, with function names as keys and the details to build an interface on the fly, e.g. along the lines:
let derivative (f : IFunction) =
match derivativeDictionary.TryGetValue(f.Name) with
| false, _ -> None
| true, d -> d.Derivative
This would require me to create one such function per feature that I add in addition to one dictionary per feature. Especially if implemented asynchronously with agents, this might be not that slow, but it still feels a little clunky.
I think the problem that you're trying to solve here is what is called The Expression Problem. You're essentially trying to write code that would be extensible in two directions. Discriminated unions and object-oriented model give you one or the other:
Discriminated union makes it easy to add new operations (just write a function with pattern matching), but it is hard to add a new kind of expression (you have to extend the DU and modify all code
that uses it).
Interfaces make it easy to add new kinds of expressions (just implement the interface), but it is hard to add new operations (you have to modify the interface and change all code that creates it.
In general, I don't think it is all that useful to try to come up with solutions that let you do both (they end up being terribly complicated), so my advice is to pick the one that you'll need more often.
Going back to your problem, I'd probably represent the function just as a function name together with the parameters:
type Expr =
| Num of decimal
| Var of string
| Application of string * Expr list
Really - an expression is just this. The fact that you can take derivatives is another part of the problem you're solving. Now, to make the derivative extensible, you can just keep a dictionary of the derivatives:
let derrivatives =
dict [ "sin", (fun [arg] -> Application("cos", [arg]))
... ]
This way, you have an Expr type that really models just what an expression is and you can write differentiation function that will look for the derivatives in the dictionary.
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.
I thought that I might be able to do this with quotations - but I can't see how.
Should I just use a table of the functions with their names - or is their a way of doing this?
Thanks.
For more info......
I'm calling a lot of f# functions from excel and I wondered if I could write a f# function
let fs_wrapper (f_name:string) (f_params:list double) =
this bit calls fname with f_params
and then use
=fs_wrapper("my_func", 3.14, 2.71)
in the sheet rather than wrap all the functions separately.
You'll need to use standard .NET Reflection to do this. Quotations aren't going to help, because they represent function calls using standard .NET MethodInfo, so you'll need to use reflection anyway. The only benefit of quotations (compared to naive reflection) is that you can compile them, which could give you better performance (but the compilation isn't perfect).
Depending on your specific scenario (e.g. where are the functions located), you'd have to do something like:
module Functions =
let sin x = sin(x)
let sqrt y = sqrt(y)
open System.Reflection
let moduleInfo =
Assembly.GetExecutingAssembly().GetTypes()
|> Seq.find (fun t -> t.Name = "Functions")
let name = "sin"
moduleInfo.GetMethod(name).Invoke(null, [| box 3.1415 |])
Unless you need some extensibility or have a large number of functions, using a dictionary containing string as a key and function value as the value may be an easier option:
let funcs =
dict [ "sin", Functions.sin;
"sqrt", Functions.sqrt ]
funcs.[name](3.1415)
There are many methods but one way is to use Reflection, for instance:
typeof<int>.GetMethod("ToString", System.Type.EmptyTypes).Invoke(1, null)
typeof<int>.GetMethod("Parse", [|typeof<string>|]).Invoke(null, [|"112"|])
GetMethod optionally takes an array of types that define the signature, but you can skip that if your method is unambiguous.
Following up on what Thomas alluded to, have a look at Using and Abusing the F# Dynamic Lookup Operator by Matthew Podwysocki. It offers a syntactically clean way for doing dynamic lookup in F#.
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/.