Conversion of lambda expressions to Func

问题

Given the following:

open System.Linq

let seqA = { 1..10 }

this works:

seqA.All (fun n -> n > 0)

However this doesn't:

let abc = fun n -> n > 0

seqA.All (abc)

Why does F# offer implicit conversion from lambda expressions to Funcs but not from functions? Pointers to the documentation where I can read up on what's going on here are welcome. :-)

回答1:

This is covered in the (rather involved) section of the spec on Method Resolution and again in Type-directed Conversions at Member Invocations. Quoting from the latter:

As described in Method Application Resolution (see §14.4), two type-directed conversions are applied at method invocations.

The first type-directed conversion converts anonymous function expressions and other function-valued arguments to delegate types. Given:

• A formal parameter of delegate type D
• An actual argument farg of known type ty₁ -> ... -> ty_n -> rty
• Precisely n arguments to the Invoke method of delegate type D

Then:

• The parameter is interpreted as if it were written:
  new D(fun arg₁ ... arg_n -> farg arg₁ ... arg_n)

It seems to suggest this conversion would be applied to any function value, but observation suggests it's applied only to anonymous functions.

来源：https://stackoverflow.com/questions/23234047/conversion-of-lambda-expressions-to-func