Or using keyword arguments: julia> MyType(; field1 = 1, field2 = 2, field3 = 3, field4 = 4, field5 = 5) = MyType(field1, field2, field3, field4, field5) MyType (constructor with 2 methods)
julia> MyType(field4 = "asdf") MyType(1,2,3,"asdf",5) You can also mix in positional arguments before the semicolon to require a few of the fields to be specified while giving the rest of them default values. Keyword arguments are often a good option for functions (not just constructors) with a whole bunch of optional arguments like this. On Sun, Dec 22, 2013 at 9:10 PM, Johan Sigfrids <[email protected]> wrote: > Why can't you use a outer constructor to apply default values for the type? > > type MyType > field1 > field2 > field3 > field4 > field5 > end > > MyType() = MyType(default1, default2, default3, default4, default5) > > Then you can instantiate an instance with MyType() and then just edit the > fields you want different. > > m = MyType() > m.field3 = 55 > m.field5 = "abc" > > > > On Sunday, December 22, 2013 12:39:17 PM UTC+2, Marcus Urban wrote: >> >> The reason that the inner constructor is written that way is to avoid >> having to assign values to every field when constructing a new instance. >> Since default values are not implemented, that approach will not work. >> >> I am trying to avoid the situation of having a composite type with, say, >> 20 fields, many of which have are reasonable defaults, and having to write m >> = myType(3, "Triangle", …, 2.5) where the ellipses represents a sequence of >> 17 values. (Is the 15th parameter the month of George Washington's birth or >> the high temperature in Alaska in 1945?) >> >> >> On Saturday, December 21, 2013 9:10:36 AM UTC-6, John Myles White wrote: >>> >>> Assigning default values to fields of a composite type is not yet >>> supported. >>> >>> Your inner constructor is also a little un-Julian, since `MyType() = >>> new()` doesn’t assign any values to those fields. >>> >>> — John >>> >>> On Dec 21, 2013, at 4:37 AM, Marcus Urban <[email protected]> wrote: >>> >>> > I am a little confused about constructing composite types. Given the >>> > definition >>> > >>> > type MyType >>> > x::Int >>> > y::Int = 6 >>> > MyType() = new() >>> > end >>> > >>> > an instance of MyType can be created using >>> > >>> > m = MyType() >>> > >>> > At that point, m.x acts as expected --- I can assign to it, read its >>> > value, and so forth. However, attempting to access m.y produces an error >>> > that MyType has no field y. Based on another post, I gather that my >>> > attempt >>> > to provide a value to m.y in this manner is not allowed If that's the >>> > case, >>> > what exactly is the effect of "y::Int = 6" If this part of the code is >>> > completely ignored, it would be really nice if the system let me know >>> > since >>> > initializing fields in this way is common in many languages. >>> > >>> > Also, I gather that a workaround is to use a constructor that takes >>> > named arguments. Is that still the recommended way? With just two fields, >>> > things are not difficult, but if the type has 20, calling a constructor >>> > with >>> > 20 positional arguments would be difficult. >>> > >>> > >>> >
