I have this type:
type ClockEvent (name: string, interval: TimeSpan, callback: unit -> unit) =
...
member this.Start () =
...
and I use it like this:
ClockEvent("autowrite", TimeSpan.FromSeconds(0.5), fun _ ->
match databaseWriterAsync connectionString |> Async.RunSynchronously with
| Ok _ -> ()
| Error e -> error $"couldn't write to the candles and kwap to the database: {e.Describe()}"
).Start()
which works as expected.
But if I add a space between 'ClockEvent' and '(', I get:
ClockEvent ("autowrite", TimeSpan.FromSeconds(0.5), fun _ ->
match databaseWriterAsync connectionString |> Async.RunSynchronously with
| Ok _ -> ()
| Error e -> error $"couldn't write to the candles and kwap to the database: {e.Describe()}"
).Start()
and, suddenly:
Writer.fs(54, 20): [FS0597] Successive arguments should be separated by spaces or tupled, and arguments involving function or method applications should be parenthesized
I understand that the issue is linked to the '.Start()' at the end, but I don't get how the space causes the compiler to interpret the code differently.
To add a bit more context, the tricky thing is that there is a subtle difference in F# between function application and method calls.
In principle, you can write them both with and without spaces:
foo(14) // Function call, but odd syntax with parentheses
foo (14) // The same, but now without space
foo 14 // This is how you typically write it
The same with method calls, but here the convention is different:
"abc".IndexOf('b') // Normal way of writing it
"abc".IndexOf 'b' // You are allowed to have a space
"abc".IndexOf 'b' // Parentheses around the 'a' expression
The problem with spaces is that F# generally requires you to parenthesize things:
foo (bar 1 (qux 2 3))
If you had to do this with method calls (this was the case in ancient versions of F#), it would be very tedious:
("foo".Substring(1)).Equals("oo") // This looks ugly
("foo".Substring 1).Equals "oo" // But you can have spaces
So, the special high-precedence application rule says that if you do not have spaces (it looks like a method call), you are not required to write parentheses:
"foo".Substring(1).Equals("oo") // Works thanks to high-precedence rule
Related
I am calling the string method "contains" in a lambda function, and would like to negate it. I thought this could be done with not myString.Contains("abbr") but it gives me the error
Successive arguments should be separated by spaces or tupled, and arguments involving function or method applications should be parenthesized
My actual function is this
open System.IO
let createWordArray filePath =
File.ReadLines(filePath)
|> Seq.filter (fun line -> line <> "")
|> Seq.filter (fun line -> not line.Contains("abbr.")) // Error occurs here
|> Seq.map (fun line -> line.Split(' ').[0])
|> Seq.filter (fun word -> word.StartsWith("-") || word.EndsWith("-"))
|> Seq.toArray
Please point out any other obvious mistakes I'm making.
You just need to add parentheses around the argument of the not function:
|> Seq.filter (fun line ->
not (line.Contains("abbr.")))
Without the parentheses, the compiler is interpreteing your code as a call to not with two arguments:
not (line.Contains) ("abbr.")
F# syntax is not like C# (or C, or C++, or Java)
In particular, F# does not use parentheses for passing function arguments. Instead, F# uses whitespace for that:
let x = f y z
You are, of course, free to enclose any terms in parentheses if you wanted to indicate the order of operations, or just for aesthetic reasons:
let x = f (y+5) z // parens for order of operations
let x = f (y) (z) // parens just for the heck of it
So you see, when you write:
line.Contains("abbr.")
There is no special meaning to the parens. You could just as well write this:
line.Contains "abbr."
It would be equivalent.
See what's happening? Not yet? Well, ok, let's try to add the not to the mix:
not line.Contains "abbr."
Is it clearer now? This looks like you're trying to call the not function, and you're giving it two arguments: first argument is line.Contains, and the second argument is "abbr."
This is not what you meant, right? What you meant was probably to first call line.Contains passing it "abbr " as argument, and then pass the result of that to not
The most straightforward way to do this is to use parentheses to indicate the order of operations:
not (line.Contains "abbr.")
Or, alternatively, you could use operator <|, which is intended specifically for this kind of thing. It just passes a parameter to a function, so pretty much does nothing. But its point is that it's an operator, so it's precedence is lower than a function call:
not <| line.Cobtains "abbr."
-module(erltoy).
-compile(export_all).
isFive(5) -> true;
isFive(_) -> false.
foo(X) ->
case X of
isFive(X) -> true;
3 -> false;
_ -> nope
end.
1> c(erltoy).
erltoy.erl:9: illegal pattern
error
Can I not call functions as part of the pattern match?
isFive(X) -> true; contains expression's which can't be computed to a constant at compile time and thus is not a valid pattern as a result. An arithmetic expression can be used within a pattern if it meets both of the following two conditions:
It uses only numeric or bitwise operators.
Its value can be evaluated to a constant when complied.
See this Example from the Erlang reference manual
case {Value, Result} of
{?THRESHOLD+1, ok} -> ...
to complete #byaruhaf answer, the left part of a case clause doesn't need to be a constant when compiled. the following code is valid, and obviously, Temp is not known at compile time (but foo(5) evaluates to nope!).
-module(erltoy).
-compile(export_all).
isFive(5) -> true;
isFive(_) -> false.
foo(X) ->
Temp = isFive(X),
case X of
Temp -> true;
3 -> false;
_ -> nope
end.
It is even not necessary that the left part is bound at execution time, for example, this is also valid, and there I is unbound before the case evaluation, and bound during the pattern matching:
get_second_element_of_3_terms_tuple_if_pos_integer(X) ->
case X of
{_,I,_} when is_integer(I), I>0 -> {true,I};
_ -> false
end.
The left part of a case must be a valid pattern with an optional guard sequence.
A valid pattern is an erlang term that may contains unbound variables, it may also contain arithmetic expressions if they respect the 2 conditions
It uses only numeric or bitwise operators.
Its value can be evaluated to a constant when complied.
the definition of a guard sequence is given there in erlang documentation
A final remark, the usual erlang way to code the kind of test function given in your example is to use different function heads, just as you do for isFive/1 definition.
I am running into difficulty with F# in numerous scenarios. I believe I'm not grasping some fundamental concepts. I'm hoping someone can track my reasoning and figure out the (probably many) things I'm missing.
Say I'm using Xunit. What I'd like to do is, provided two lists, apply the Assert.Equal method pairwise. For instance:
Open Xunit
let test1 = [1;2;3]
let test2 = [1;2;4]
List.map2 Assert.Equal test1 test2
The compiler complains that the function Equal does not take one parameter. As far as I can tell, shouldn't map2 be providing it 2 parameters?
As a sanity check, I use the following code in f# immediate:
let doequal = fun x y -> printf "result: %b\n" (x = y)
let test1 = [1;2;3]
let test2 = [1;2;4]
List.map2 doequal test1 test2;;
This seems identical. doequal is a lambda taking two generic parameters and returning unit. List.map2 hands each argument pairwise into the lambda and I get exactly what I expected as output:
result: true
result: true
result: false
So what gives? Source shows Xunit.Equal has signature public static void Equal<T>(T expected, T actual). Why won't my parameters map right over the method signature?
EDIT ONE
I thought two variables x and y vs a tuple (x, y) could construct and deconstruct interchangeably. So I tried two options and got different results. It seems the second may be further along than the first.
List.map2 Assert.Equal(test1, test2)
The compiler now complains that 'Successive arguments should be separated spaces or tupled'
List.map2(Assert.Equal(test1, test2))
The compiler now complains that 'A unique overload method could not be determined... A type annotation may be needed'
I think that part of the problem comes from mixing methods (OO style) and functions (FP style).
FP style functions have multiple parameters separated by spaces.
OO style methods have parens and parameters separated by commas.
Methods in other .NET libraries are always called using "tuple" syntax (actually subtly different from tuples though) and a tuple is considered to be one parameter.
The F# compiler tries to handle both approaches, but needs some help occasionally.
One approach is to "wrap" the OO method with an FP function.
// wrap method call with function
let assertEqual x y = Assert.Equal(x,y)
// all FP-style functions
List.map2 assertEqual test1 test2
If you don't create a helper function, you will often need to convert multiple function parameters to one tuple when calling a method "inline" with a lambda:
List.map2 (fun x y -> Assert.Equal(x,y)) test1 test2
When you mix methods and functions in one line, you often get the "Successive arguments should be separated" error.
printfn "%s" "hello".ToUpper()
// Error: Successive arguments should be separated
// by spaces or tupled
That's telling you that the compiler is having problems and needs some help!
You can solve this with extra parens around the method call:
printfn "%s" ("hello".ToUpper()) // ok
Or sometimes, with a reverse pipe:
printfn "%s" <| "hello".ToUpper() // ok
The wrapping approach is often worth doing anyway so that you can swap the parameters to make it more suitable for partial application:
// wrap method call with function AND swap params
let contains searchFor (s:string) = s.Contains(searchFor)
// all FP-style functions
["a"; "b"; "c"]
|> List.filter (contains "a")
Note that in the last line I had to use parens to give precedence to contains "a" over List.filter
public static void Equal<T>(T expected, T actual)
doesn't take two parameters - it takes one parameter, which is a tuple with two elements: (T expected, T actual).
Try this instead:
List.map2 Assert.Equal(test1, test2)
It's all there in the type signatures.
The signature for Assert.Equals is something along the lines of 'a * 'a -> unit. List.map2 expects a 'a -> 'b -> 'c.
They just don't fit together.
List.map2 (fun x y -> Assert.Equal(x,y)) test1 test2 - works because the lambda wrapping Equals has the expected signature.
List.zip test1 test2 |> List.map Assert.Equal - works because you now have a single list of tuples, and since List.map wants an 'a -> 'b function (where 'a is now a tuple), Assert.Equal is now fair game.
It's simply not true that two values and a tuple are implicitly interchangeable. At least not as far as F# the language is concerned, or the underlying IL representation is concerned. You can think that it's that way when you call into F# code from, say, C# - an 'a -> 'b -> 'c function there is indeed called the same way syntactically as an 'a * 'b -> 'c function - but this is more of an exception than a rule.
According to its signature Xunit.Assert.Equal() takes a single 2 values tuple parameter
I'm learning F# by writing a recursive descent parser using active patterns.
Since all my rules or partial active patterns I need to combine them in different manners, but I'm getting really frustrated with the syntax of passing active patterns as parameters.
The following example shows the trouble I'm having:
// Combines two patterns by chaining them.
let (|Chain|_|) (|Pattern1|_|) (* Should I use pipes here? *) (|Pattern2|_|) data =
match data with
|Pattern1 result ->
match result with
|Pattern2 result2 -> Some result2
|_ -> None
|_ -> None
// Stupid test patterns
let (|IfBiggerThan10ThenDouble|_|) value = if value > 10 then Some (value*2) else None
let (|IfLessThan100ThenDouble|_ |) value = if value < 100 then Some (value*2) else None
match 20 with
// Do I need pipes here?
|Chain (IfBiggerThan10ThenDouble IfLessThan100ThenDouble) value -> printfn "%A" value // Should print 80
| _ -> printfn "Did not match"
My main confusion seems to be about the '|' operator. Sometimes it seems to be a part of the type of the pattern and sometimes part of the name.
You do not really need to implement your own chaining of patterns, because you can directly nest the patterns which gives you the required result:
match 20 with
| IfBiggerThan10ThenDouble(IfLessThan100ThenDouble value) -> printfn "%A" value
| _ -> printfn "Did not match"
This will first call the IfBiggerThan10ThenDouble pattern which calculates 20*2 and passes the value to the nested pattern IfLessThan100ThenDouble. This again doubles the value and binds it to the value symbol (when it succeeds).
That said, your implementation of the Chain pattern actually works and can be called like this:
match 20 with
| Chain (|IfBiggerThan10ThenDouble|_|) (|IfLessThan100ThenDouble|_|) value ->
printfn "%A" value // Should print 80
| _ -> printfn "Did not match"
In general, active pattern (|P|_|) is really just a function with a special name. You can treat it as an ordinary function and call it by writing (|P|_|) argument or you can treat it as a value and pass it as an argument to other functions or parameterized active patterns. Your code would work if you implemented Chain as a pattern taking ordinary functions:
let (|Chain|_|) f g data =
f data |> Option.bind (fun r -> g data)
Then Chain <arg1> <arg2> <pat> is just calling the parameterized active pattern with two functions as an argument. When called, it binds the result to the pattern <pat>. In the above example, the two arguments are function values representing the patterns (these could be ordinary functions, but not lambda functions because of syntactic restrictions).
How can I write a no-op statement in F#?
Specifically, how can I improve the second clause of the following match statement:
match list with
| [] -> printfn "Empty!"
| _ -> ignore 0
Use unit for empty side effect:
match list with
| [] -> printfn "Empty!"
| _ -> ()
The answer from Stringer is, of course, correct. I thought it may be useful to clarify how this works, because "()" insn't really an empty statement or empty side effect...
In F#, every valid piece of code is an expression. Constructs like let and match consist of some keywords, patterns and several sub-expressions. The F# grammar for let and match looks like this:
<expr> ::= let <pattern> = <expr>
<expr>
::= match <expr> with
| <pat> -> <expr>
This means that the body of let or the body of clause of match must be some expression. It can be some function call such as ignore 0 or it can be some value - in your case it must be some expression of type unit, because printfn ".." is also of type unit.
The unit type is a type that has only one value, which is written as () (and it also means empty tuple with no elements). This is, indeed, somewhat similar to void in C# with the exception that void doesn't have any values.
BTW: The following code may look like a sequence of statements, but it is also an expression:
printf "Hello "
printf "world"
The F# compiler implicitly adds ; between the two lines and ; is a sequencing operator, which has the following structure: <expr>; <expr>. It requires that the first expression returns unit and returns the result of the second expression.
This is a bit surprising when you're coming from C# background, but it makes the langauge surprisingly elegant and consise. It doesn't limit you in any way - you can for example write:
if (a < 10 && (printfn "demo"; true)) then // ...
(This example isn't really useful - just a demonstration of the flexibility)