I'm new to F# and I'm trying a few thing to get my head around the language.
I have to two types almost identical (Coordinate and Vector). Because of this type inference cannot work properly and I have hard time to specify the correct type on each function.
It somehow undestand that's a Vector here:
type Coordinate = {X:int; Y:int}
type Vector = {X:int; Y:int}
let calculateVector (origin:Coordinate) (destination:Coordinate) = { X=destination.X-origin.X; Y= destination.Y-origin.Y;}
And here when I want a return type of Coordinate, I cannot find how to specify the return it for this function:
let calculateNextCoordinate (coordinate:Coordinate) direction =
match direction with
| "N" -> { X=coordinate.X; Y=coordinate.Y-1 }
| "NE" -> { X=coordinate.X+1; Y=coordinate.Y-1 }
| "E" -> { X=coordinate.X+1; Y=coordinate.Y }
| "SE" -> { X=coordinate.X+1; Y=coordinate.Y+1 }
| "S" -> { X=coordinate.X; Y=coordinate.Y+1 }
| "SW" -> { X=coordinate.X-1; Y=coordinate.Y+1 }
| "W" -> { X=coordinate.X-1; Y=coordinate.Y }
| "NW" -> { X=coordinate.X-1; Y=coordinate.Y-1 }
| _ -> coordinate
I have this error on the default case: This expression was expected to have 'Vector' but here has type 'Coordinate'
i tired to have a look on this website for function signatures but could not something for my problem: https://fsharpforfunandprofit.com/posts/function-signatures/
Questions:
How do you fix this error?
Is it because inference type take by default the last type declared that match the properties (in my example Vector)?
Bonus: Is there a better way to handle this kind of situation in F#?
Thanks in advance
Since records are constructed using their member names:
{ X = 2; Y = 3}
you created a naming conflict between Vector and Coordinate. In F# the compiler always resolves to the latest definition in such cases and therefore, in your example, the compiler will interpret the record { X = ..., Y = ...} as a Vector.
There is a good article on F# records on fsharpforfunandprofit.com, that explains how to handle this easily, which I suggest you read for a good explanation.
But in short, you can prefix either member of the record instance with the record type:
{ Coordinate.X = 2; Y = 3 } // Creates a Coordinate
{ X = 2; Coordinate.Y = 3 } // Creates a Coordinate
and
{ Vector.X = 2; Y = 3 } // creates a vector
Related
Given the follwing code:
type Creature =
{ Strength: int
Toughness: int }
type CardType =
| Creature of Creature
| Land
| Instant
type Card =
{ Types: CardType list }
module Card =
let isType t card = List.contains t card.Types
I am able to write
Card.isType Land
When trying to check if card is a creature, i get the following error:
This expression was expected to have type
'CardType'
but here has type
'Creature -> CardType'
Is it even possible to have a "isType" function like this or am I stuck with pattern matching on a separate "isCreature" function instead?
Unless you want to resort to various reflection-based hacks, you are stuck with pattern matching. I would probably define a bit more general function using List.exist rather than List.contains (taking a predicate). Then you can easily define three functions for your specific card types:
module Card =
let isType t card =
List.exists t card.Types
let isCreature =
isType (function Creature _ -> true | _ -> false)
let isLand = isType ((=) Land)
let isInstant = isType ((=) Instant)
For Land and Instant, you can just check if the value equals the specific one you're looking for. For Creature, this requires pattern matching - but can be done quite nicely using function.
I have a data pipeline where at each step more data fields are required. I would like to do this in a functional way by respecting immutability. I could achieve this with a class by I am wondering if there is an F# way of doing it?
// code that loads initial field information and returns record A
type recordA = {
A: int
}
// code that loads additional field information and returns record AB
type recordAB = {
A: int
B: int
}
// code that loads additional field information and returns record ABC
type recordABC = {
A: int
B: int
C: int
}
As records are sealed I can't just inherit them. How can I avoid having to define a new record with the exact same fields as the previous step and adding the required fields? Preferably I would like to have something like one record that has all required fields and the fields get assigned to their values in each step.
Note that the number of fields added in each step could be more than 1.
I think this can be a good use case for the anonymous records recently introduced in F#.
let a = {| X = 3 |}
let b = {| a with Y = "1"; Z = 4.0|}
let c = {| b with W = 1 |}
printfn "%d, %s, %f, %d" c.X c.Y c.Z c.W
One way to do it in a very FP-style would be to use a DU with a case for each step of the workflow, and the appropriate data for each step in each case:
type WorfklowState =
| StepOne of int
| StepTwo of int * int
| StepThree of int * int * int
Then your entire workflow state, both what step you're currently on and the data produced/consumed by that step, would be modeled in the data type. Of course, you would probably create record types for the data of each case, rather than using progressively larger tuples.
Depending on the application, this may be a (mis-)use case for a dynamic data container.
F# might help by providing user-defined dynamic lookup operators, for which a special syntactic translation occurs.
let (?) (m : Map<_,_>) k = m.Item k
// val ( ? ) : m:Map<'a,'b> -> k:'a -> 'b when 'a : comparison
let (?<-) (m : Map<_,_>) k v = m.Add(k, v)
// val ( ?<- ) : m:Map<'a,'b> -> k:'a -> v:'b -> Map<'a,'b> when 'a : comparison
let m = Map.empty<_,_>
let ma = m?A <- "0"
let mabc = (ma?B <- "1")?C <- "2"
ma?A // val it : string = "0"
mabc?C // val it : string = "2"
You can "inherit" records:
type RecordA =
{
a : int
}
type RecordAB =
{
a : RecordA
b : int
}
type RecordABC =
{
ab : RecordAB
c : int
}
Then you can access all of the elements, though with longer and longer chain as you go deeper and deeper.
However, why don't you just use a list of elements to store the result?
First, I would create a type to handle all possible types that you may have on each step, e.g.:
type Step =
| Int of int
| String of string
// ...
Then you can represent the workflow simply as:
type WorkflowState = list<Step>
and if you want to ensure that you always have at least one element then you can use:
type WorkflowState = Step * list<Step>
However, the records have labels and the structure above does not have them! So, if you do need labels, then you can represent them by a map using either a strong type:
type Label =
| A
| B
// ...
type WorkflowMappedState = Map<Label, Step>
or just a string based one, e.g.
type WorkflowMappedState = Map<string, Step>
The benefits of list or map based approach in comparison to the answers above is that you don't have to know the maximum number of possible steps. What if the number of steps is over 100? Would you want to create a record with 100+ labels? Most likely not! The anonymous records are great, but what if you want to use them outside of module where they were created? I think that that would cause some troubles.
Having said all that, I think that I would go with a list based approach: type WorkflowState = list<Step>. It is very F# way and it is very easy to transform further.
This example code:
type recordA = { X: string; }
type recordB = { X: string; }
let modifyX newX record = { record with X = newX }
let modifiedRecordA = {recordA.X = "X"} |> modifyX "X2"
let modifiedRecordB = {recordB.X = "X"} |> modifyX "X2"
Results in:
let modifyX newX record = { record with X = newX }
--------------------------^^^^^^^^^^^^^^^^^^^^^^^^
stdin(4,27): warning FS0667: The field labels and expected type of this record expression or pattern do not uniquely determine a corresponding record type
let modifiedRecordA = {recordA.X = "X"} |> modifyX "X2"
-------------------------------------------^^^^^^^^^^^^
stdin(6,44): error FS0001: Type mismatch. Expecting a
recordA -> 'a
but given a
recordB -> recordB
The type 'recordA' does not match the type 'recordB'
My expectation is that modifiedRecordA ends up equivalent to { recordA.X = "X2" } and modifiedRecordB ends up equivalent to { recordB.X = "X2" }, but it doesn't seem to work that way.
Why doesn't this just infer and return the appropriate record type based on the parameter type?
Is there anything I can do to make this work?
The inline magic required to make this work is based on overload resolution combined with statically resolved member constraints. Overloads defined as operators avoid the need to spell out explicit constraints.
type Foo = Foo with
static member ($) (Foo, record) = fun newX -> { record with recordA.X = newX }
static member ($) (Foo, record) = fun newX -> { record with recordB.X = newX }
let inline modifyX newX record = (Foo $ record) newX
let modifiedRecordA = {recordA.X = "X"} |> modifyX "X2"
let modifiedRecordB = {recordB.X = "X"} |> modifyX "X2"
Constructs which pass types for which no overload exists do not compile.
type recordC = { X: string; }
let modifiedRecordC = {recordC.X = "X"} |> modifyX "X2"
// Error No overloads match for method 'op_Dollar' ...
// Possible overload ... The type 'recordC' is not compatible with the type 'recordB'
// Possible overload ... The type 'recordC' is not compatible with the type 'recordA'
This is not really intended for actual use. Heed the advise and explore if other approaches are better suited to your problem.
the function modifyX is incorrect. You may not use the term X in the definition and have that X refer to different fields.
Section 6.3.6 of the F# spec
Each field label must resolve to a field Fi in a single record type R, all of whose fields are accessible.
By passing recordA and recordB to modifyX the X is not uniquely determined to be a field of a single type.
What you actually want is probably a polymorphic property member inherited through an interface, not a set of record types with a common member name.
The problem is that the compiler is inferring the type of modifyX based on usage. My understanding is that this proceeds from the bottom up, so the type is inferred to be val modifyX : newX:string -> record:recordB -> recordB. This, of course, then results in a type error when attempting to use this with a record of type recordA. The warning is telling you that, although its picking a type, there is another type with the same fields, so all the compiler can do is make its best guess at which type you meant. It might be possible to achieve what you're trying to do with inline functions, but I'm not sure how that might work offhand.
Is it possible to express something like this:
type id = int > 0
I know its not possible to do statically, since this would mean F# has dependent types. In C# I'm used to do this sort of thing with code contracts and get a runtime enforcement. I'm looking for something similiar here.
Thanks
EDIT:
Thank you for all the answers which have various pros and cons. At the monent I'm only using a small subset of F#, a subset of the ocaml core that lends itself easily to program proofs. So no classes.
Contrary to what others said, I would suggest not using classes here, if I understood your problem correctly.
Since the value is immutable, we need applying constraint only once. Any wrapper classes would be an overhead and load GC. Instead, a simple function will do the job:
let inline constrained predicate errormessage value =
if not (predicate value)
then invalidArg "value" errormessage
else value
let positive =
constrained (fun x -> x > 0) "Value must be positive"
let int1 = positive 5 // OK
let int2 = positive -3 // ArgumentException
You can do the same for other types:
let mustBeLong =
constrained (fun (x:string) -> x.Length > 3) "String must be long"
let str1 = mustBeLong "foobar" // OK
let str2 = mustBeLong "baz" // ArgumentException
Using the same within a struct:
type Point2D =
struct
val X: int
val Y: int
new(x: int, y: int) = { X = positive x; Y = positive y }
end
let point1 = Point2D(5, 3) // OK
let point2 = Point2D(5, -2) // ArgumentException
Define it as a union type:
type Id = Id of int
and shadow the constructor with another function:
let Id n =
assert(n > 0)
Id n
In F#, you have to resort to classes and check arguments inside constructors. Other types such as discriminated unions, records and structs have implicit constructors which you can't easily alter.
type Id(i: int) =
do if i <= 0 then
invalidArg "i" "the argument has to be a positive integer"
member x.Value = i
Pattern matching doesn't play nicely with classes. You can remedy the problem using active patterns:
let (|Id|) (id: Id) = id.Value
let id = Id(1)
match id with
| Id 1 -> printfn "matched"
| _ -> printfn "unmatched"
You could create a generic class like so:
type verify<'t>(t:'t,cond) =
let mutable tval = t
let _verify v = if not (cond v) then failwith "bad argument"
do _verify tval
member x.get() = tval
member x.set v =
_verify v
tval <- v
then you can use it with
verify(1,fun t -> t>0)
using .set will recheck the condition.
The active pattern in this question fails to compile after upgrading to VS 2012 RTM. It provides a way to do a type test and match a literal within a single pattern. For example:
let (|Value|_|) value =
match box value with
| :? 'T as x -> Some x
| _ -> None
let getValue (name: string) (r: IDataReader) =
match r.[name] with
| null | :? DBNull | Value "" -> Unchecked.defaultof<_>
| v -> unbox v
Can this be done without the active pattern? I realize a when guard could be used (:? string as s when s = "") but it can't be combined with other patterns.
kvb's variation (which doesn't do quite the same thing since it assumes the type test succeeds) can be modified to produce a similar pattern:
let (|Value|_|) x value =
match box value with
| :? 'T as y when x = y -> Some()
| _ -> None
However, there is a subtle performance difference. The original active pattern translates to:
public static FSharpOption<T> |Value|_|<a, T>(a value)
{
object obj = value;
if (!LanguagePrimitives.IntrinsicFunctions.TypeTestGeneric<T>(obj))
{
return null;
}
return FSharpOption<T>.Some((T)((object)obj));
}
that is, it does a type test and cast. It's usage (match x with Value "" -> ...) translates to:
FSharpOption<string> fSharpOption = MyModule.|Value|_|<object, string>(obj);
if (fSharpOption != null && string.Equals(fSharpOption.Value, ""))
{
...
}
Most notably, the typed value returned from the pattern is matched using the typical compiler transformations for patterns (string.Equals for strings).
The updated pattern translates to:
public static FSharpOption<Unit> |Value|_|<T, a>(T x, a value)
{
object obj = value;
if (LanguagePrimitives.IntrinsicFunctions.TypeTestGeneric<T>(obj))
{
T y = (T)((object)obj);
T y3 = y;
if (LanguagePrimitives.HashCompare.GenericEqualityIntrinsic<T>(x, y3))
{
T y2 = (T)((object)obj);
return FSharpOption<Unit>.Some(null);
}
}
return null;
}
which uses generic equality and is less efficient than matching against a literal. The usage is a bit simpler since equality is baked into the pattern:
FSharpOption<Unit> fSharpOption = MyModule.|Value|_|<string, object>("", obj);
if (fSharpOption != null)
{
...
}
Anyway, it works. But I like the original better.
You should be able to use a parameterized active pattern:
let (|Value|_|) v x =
if unbox x = v then
Some()
else None
The usage should look exactly like what you've got now.
Edit
While I don't know if the breaking change was intentional, I believe that active patterns with generic return types unrelated to the input types should usually be avoided. When combined with type inference, they can easily mask subtle errors. Consider the following example, using your original (|Value|_|) pattern:
match [1] with
| Value [_] -> "Singleton"
| _ -> "Huh?"
It seems like this isn't something you would actually ever attempt - the name implies that Value should only be used with literals; parameterized active patterns enable exactly this scenario.