18. Scala Language Specification (2.9.0)
7.3 Views(p.193) より引用
• Implicit parameters and methods can also
define implicit conversions called views.
• A view from type S to type T is defined by
an implicit value which has function type
S=>T or (=>S)=>T or by a method
convertible to a value of that type.
19. implicit def ... でconversionを定義できるのは
method type: (A) B function type: (A => B)
への変換が可能だと言語仕様で規定されているため