scala - What's the difference between these two ways to declare functions? -

follow 2 similar functions, a , b, declared differently:

scala> val = { x:int => x+x } a: int => int = <function1>  scala> def b(x:int) = x+x b: (x: int)int 

the difference can find between 2 def function can specify parameter's name pass (e.g. b(x=1)). how these functions different in other regards? should used when?

edit:

i not able use @tailrec annotation val method.

the principal difference in how scala translates definitions. in scala lambda (an anonymous function, function value, function literal) , method different things. first definition of yours sugar instance of function1 class, 1 of classes used lambdas, second method of surrounding object, compiles standard java method.

the practical difference between 2 scala cannot let refer method value, renders second class citizen. results in redundant indirections when dealing higher order functions. e.g., following:

1.to(4).map(b) 

desugars wrapper lambda:

1.to(4).map((x: int) => b(x)) 

which sugar for:

1.to(4).map(   new function1[int, int] {     def apply(x: int): int = b(x)   } ) 

feels quite redundancy, right? why if intention function used first class citizen (i.e., being passed around value) more reasonable declare lambda beginning.