Multifactor activated templates returning FS0722 error Only active templates returning exactly one result can accept arguments
Since I just found the Japanese pages about this error, I thought, let me log and ask here as my Japanese variety is rusty.
If I have the following active FSharp template (simplified example):
let (|InRange|OutOfRange|) from too =
function
| input when input >= from && input <= too -> InRange
| _ -> OutOfRange
It compiles fine and shows its type as:
val ( |InRange|OutOfRange| ) :
from:'a -> too:'a -> _arg1:'a -> Choice<unit,unit> when 'a : comparison
But when I try to use it, that is, as follows, it throws an error:
let test i = match i with
| InRange 10 20 -> "in range"
| _ -> "out of range"
Throws: error FS0722: only active templates returning exactly one result can take arguments
I can resolve this by turning it into two one parameter active patterns, each returning None / Some (x), but I am still wondering why I am not allowed to do this and / or is there a syntax I can use that I am not I know. I am also wondering why it compiles, but I cannot use it?
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The simplest solution would be to refactor this into a partial active template :
let (|InRangeInclusive|_|) lo hi x =
if lo <= x && x <= hi then Some () else None
Then you can even combine them like this:
let test i = match i with
| InRangeInclusive 10 20 -> "in first range"
| InRangeInclusive 42 100 -> "in second range"
| _ -> "out of range"
Note that I took it upon myself to give the template a better name, because those who will be using your code might be confused about its corner case behavior.
I'm still wondering why I am not allowed to do this?
Why can't non-partial active templates be parameterized in F #?
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