How to pass string concatenation operator (I do not want to use ^) to function?
If I try to create variable:
let x = (+) compiler says x is of type int -> int -> int. How can I annotate x so it works on strings?
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1 Answer
UPDATED QUESTION: To answer your updated question - the + operator is generic and uses static member constraints to work on any type that supports it. This gets lost if you use it in a function or assign it to variable. The one way to keep this property is to define an inline function.
let x1 = (+) // Type constrained, because it's not a function
let x2 a b = (+) // Type constrained, because it's not `inline`
let inline x3 = (+) // Error: only functions can be `inline`
let inline x4 a b = a + b // Works and you can use `x4` with strings and ints
ORIGINAL ANSWER: There must be something else going on in your situation that you are not showing in your question. If you just open a new F# script file, the following works:
let funInt (f:int -> int -> int) = f 1 2
let funStr (f:string -> string -> string) = f "hi" "there"
funInt (+)
funStr (+)
Now, I'm not sure what else you have in your code that is causing the error. One possibility I can think of is if you re-defined the + operator - if you have a new definition that is not inline, you'll get the error you describe:
let funInt (f:int -> int -> int) = f 1 2
let funStr (f:string -> string -> string) = f "hi" "there"
let (+) a b = a + b
funInt (+)
funStr (+)
Can you post a minimal reproducible example that illustrates the issue?
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Shadow
Thomas now I see my sample code had no sense, because compiler know that
funStr works on strings. I wanted to know how to get string version of + operator, so compiler correctly types my function - updated question to reflect that.