# [julia-users] Re: redefining Base method for `show`

```I'm thinking of symbolic mathematics (Symata.jl). Another example is
`SymPy.jl`, which prints rationals the same way I want to, like this:
"2/3". But in SymPy.jl, rationals are not `Rational`'s, but rather wrapped
python objects, so the problem with printing does not arise. If I wrapped
`Rational`'s it would introduce a lot of complexity.```
```
So far, walking expression trees to wrap objects of certain types just
before printing seems to be working well.

On Friday, October 14, 2016 at 4:23:27 AM UTC+2, Jeffrey Sarnoff wrote:
>
> I assume you meant x::T in type A{T}.  Why do you want to do this:
>
>> Every time a Rational or Symbol or Bool is encountered on any level, I
>> want it to print differently than Base.show does it.
>>
> Do you want to adorn it (like "3//5" -> "{3//5}") or alter it (like "3//5"
> -> "2//5")?
>
> Also, I think you are approaching solving your problem in way more suited
> to another language.  But I really have no idea what your motivation is.
>
> On Tuesday, October 11, 2016 at 7:21:35 PM UTC-4, lapeyre....@gmail.com
> wrote:
>>
>> To make it concrete, I have
>>
>> type A{T}
>>    x
>>    a::Array{Any,1}
>> end
>>
>> The elements of the array a are numbers, Symbols, strings, etc., as well
>> as more instances of type A{T}.  They
>> may be nested to arbitrary depth. If I call show on an instance of A{T},
>> then show will be called recursively
>> on all parts of the tree. Every time a Rational or Symbol or Bool is
>> encountered on any level, I want it to print differently than Base.show
>> does it.
>>
>>
>> On Tuesday, October 11, 2016 at 11:48:46 PM UTC+2, Jeffrey Sarnoff wrote:
>>>
>>> Are you saying  a and b and c and d?
>>>
>>> (a) that you have a outer type which has a Rational field and has
>>> another field of a type that has a field which is typed Rational or is
>>> typed e.g. Vector{Rational}
>>>
>>> (b) and displaying a value of the outer type includes displaying the
>>> Rationals from withiin the field of the inner type
>>>
>>> (c) and when displaying that value, you want to present the outer type's
>>> Rational field a special way
>>>
>>> (d) and when displaying that value, you want to present the Rational
>>> fields of the inner type in the usual way
>>>
>>>
>>> On Tuesday, October 11, 2016 at 1:23:37 PM UTC-4, lapeyre....@gmail.com
>>> wrote:
>>>>
>>>> I think I understand what you are saying (not sure).  A problem that
>>>> arises is that if I call show or print on an object, then show or print
>>>> may
>>>> be called many times on fields and fields of fields, etc., including from
>>>> within Base code before the call returns. I don't know how to tell the
>>>> builtin julia code my preference for printing rationals. The only way I
>>>> know to get a redefinition of show eg M.show to work in all situations, is
>>>> to copy all the code that might be called. Maybe I'm missing something,
>>>> but
>>>> I can't see a way around this.
>>>>
>>>> I'm not familiar with the idea of a fencing module.
>>>>
>>>> On Monday, October 10, 2016 at 11:30:18 PM UTC+2, Jeffrey Sarnoff wrote:
>>>>>
>>>>> You could wrap your redefinitions in a module M without exporting show
>>>>> explicitly.
>>>>> `using M`  and accessing the your variation as `M.show` may give the
>>>>> localization you want.
>>>>> Should it not, then doing that within some outer working context, a
>>>>> fencing module, may add enough flexibility.
>>>>>
>>>>>
>>>>> On Monday, October 10, 2016 at 4:18:52 PM UTC-4, lapeyre....@gmail.com
>>>>> wrote:
>>>>>>
>>>>>> For the record, a workable solution, at least for this particular
>>>>>> code: I pass all output through wrapout() at the outermost output call.
>>>>>> The
>>>>>> object to be printed is traversed recursively. All types fall through
>>>>>> except for the handful that I want to change. Each of these is each
>>>>>> wrapped
>>>>>> in a new type. I extend Base.show for each of these wrapper types. This
>>>>>> seems pretty economical and  robust and works across versions. The
>>>>>> wrapper
>>>>>> types are only introduced upon output, the rest of the code never sees
>>>>>> them.
>>>>>>
>>>>>> This works because the code uses a customization of the REPL, and
>>>>>> several instances of print, warn, string, etc. I make a new top-level
>>>>>> output function for the REPL that uses `wrapout`. I also  generate
>>>>>> wrappers
>>>>>> for each of print, etc. that map `wrapout` over all arguments. A
>>>>>> developer
>>>>>> is expected to use the interface provided rather than `print` etc.
>>>>>> directly.  The user doesn't even have a choice. There are very few types
>>>>>> in
>>>>>> the package, but a lot of nested instances. So there is very little code
>>>>>> needed to traverse these instances.  This might be more difficult in a
>>>>>> different situation if many methods for wrapout() were required.
>>>>>>
>>>>>> On Monday, October 10, 2016 at 12:20:50 AM UTC+2,
>>>>>> lapeyre....@gmail.com wrote:
>>>>>>>
>>>>>>> I want to change show for Symbol, Rational, and Bool. Till now, I
>>>>>>> simply overwrote the existing methods. This works great. I get what I
>>>>>>> want,
>>>>>>> even in warn and error and string interpolation, etc. It works
>>>>>>> flawlessly
>>>>>>> on v0.4, v0.5, and v0.6. But, this changes the behavior for everyone.
>>>>>>> So, I
>>>>>>> want to get the same effect through different means that do not affect
>>>>>>> other code. Any idea about how to do this ?
>>>>>>>
>>>>>>> One solution is to copy a large amount of  base code that uses
>>>>>>> print, write, show, etc. renaming these functions. Other than changing
>>>>>>> 15
>>>>>>> or 20 lines for Symbol, etc., the code is unchanged. This is works more
>>>>>>> or
>>>>>>> less, but is obviously a fragile, bad solution.
>>>>>>>
>>>>>>> Another idea is to make a subtype of IO that wraps subtypes of IO. I
>>>>>>> am allowed to write methods for show for this new type. This is turning
>>>>>>> out
>>>>>>> to be complicated and unworkable.
>>>>>>>
>>>>>>> Another vague idea is to make a new mime type. Another is to copy
>>>>>>> IOStream as another type, so that I only have to write methods for
>>>>>>> Symbol,
>>>>>>> Rational and Bool again.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>```