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This produces an anonymous function, as you would expect (f is a function with three arguments):
f(_, _, _) What I don't understand is why this doesn't compile, instead giving a "missing parameter type" error:
f(_, _, 27) Instead, I need to specify the types of the underscores explicitly. Shouldn't Scala be able to infer them given that it knows what the function f's parameter types are?
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The Partially applied functions are the functions which are not applied on all the arguments defined by the stated function i.e, while invoking a function, we can supply some of the arguments and the left arguments are supplied when required.
=> is syntactic sugar for creating instances of functions. Recall that every function in scala is an instance of a class. For example, the type Int => String , is equivalent to the type Function1[Int,String] i.e. a function that takes an argument of type Int and returns a String .
Scala - Functions with Named Arguments Named arguments allow you to pass arguments to a function in a different order. The syntax is simply that each argument is preceded by a parameter name and an equals sign. Try the following program, it is a simple example to show the functions with named arguments.
References below are to the Scala Language Specification
Consider the following method:
def foo(a: Int, b: Int) = 0 Eta Expansion can convert this to a value of type (Int, Int) => Int. This expansion is invoked if:
a) _ is used in place of the argument list (Method Value (§6.7))
val f = foo _ b) the argument list is omitted, and expected type of expression is a function type (§6.25.2):
val f: (Int, Int) => Int = foo c) each of the arguments is _ (a special case of the 'Placeholder Syntax for Anonymous Functions' (§6.23))
val f = foo(_, _) The expression, foo(_, 1) doesn't qualify for Eta Expansion; it just expands to (a) => foo(a, 1) (§6.23). Regular type inference doesn't attempt to figure out that a: Int.
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If you are thinking about partial application, I thought that this was only possible with multiple parameter lists (whereas you only have one):
def plus(x: Int)(y: Int) = x + y //x and y in different parameter lists val plus10 = plus(10) _ //_ indicates partial application println(plus10(2)) //prints 12 Your example is interesting though as I was completely unaware of the syntax you describe and it appears you can have partial application with a single parameter list:
scala> def plus2(x: Int, y: Int) = x + y plus2: (x: Int,y: Int)Int scala> val anon = plus2(_,_) anon: (Int, Int) => Int = <function2> scala> anon(3, 4) res1: Int = 7 So the compiler can clearly infer the type Int!
scala> val anon2 = plus2(20,_) <console>:5: error: missing parameter type for expanded function ((x$1) => plus2(20, x$1)) val anon2 = plus2(20,_) ^ Hmmm, strange! I don't seem to be able to do partial application with a single parameter list. But then if I declare the type of the second parameter, I can have partial application!
scala> val anon2 = plus2(20,_: Int) anon2: (Int) => Int = <function1> scala> anon2(24) res2: Int = 44 EDIT - one thing I would observe is that it seems like the following two shortenings are equivalent, in which case it's a bit more obvious that this is not a "partial application" but more like a "function pointer"
val anon1 = plus2(_,_) val anon2 = plus2 _ If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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