# Custom pretty printing post-processing

**URL:** <https://discourse.julialang.org/t/custom-pretty-printing-post-processing/78844>\
**Category:** General Usage\
**Tags:** latex\
**Created:** [April 1, 2022, 1:34am UTC](https://discourse.julialang.org/t/custom-pretty-printing-post-processing/78844 "2022-04-01T01:34:20Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![jedforrest](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jedforrest/32/37502_2.png) [@jedforrest](https://discourse.julialang.org/u/jedforrest)\
**Post date:** [April 1, 2022, 1:34am UTC](https://discourse.julialang.org/t/custom-pretty-printing-post-processing/78844/1 "2022-04-01T01:34:21Z")

</div>

I want to modify a [custom pretty printing](https://docs.julialang.org/en/v1/manual/types/#man-custom-pretty-printing) function for a type defined by another package. However, I don’t want to override their printing function, I just want to **post-process** the string before it is printed.

In particular, I want to take their latex output, modify it somehow, then print it as a latex string again (in say IJulia).

Here’s what I’ve tried so far:

```julia
function Base.show(io::IO, ::MIME"text/latex", x::MyType) 
    ltx = repr("text/latex", x)
    ltx = replace(ltx, "abc" => "def")
    print(io, ltx)
end

```

This however leads to a StackOverflowError, as I believe `repr` must call `Base.show` somewhere, leading to an infinite loop. I’m not sure how else to get the output before it is printed.

Any idea on how I can do this?

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<div class="post-metadata">

**Author:** ![mcabbott](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mcabbott/32/6603_2.png) [@mcabbott](https://discourse.julialang.org/u/mcabbott)\
**Post date:** [April 1, 2022, 3:36am UTC](https://discourse.julialang.org/t/custom-pretty-printing-post-processing/78844/2 "2022-04-01T03:36:56Z")

</div>

I think you want `invoke`, which you can use to call the old method, despite defining a new and more specialised one. Something like this:

```julia
function Base.show(io::IO, m::MIME"text/latex", x::MyType)
  io = IOBuffer()
  invoke(show, Tuple{IO, MIME"text/latex", Any}, io, m, x) # need the exact signature here
  ltx = String(take!(io))
  ltx = replace(ltx, "abc" => "def")

```

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<div class="post-metadata">

**Author:** ![jedforrest](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jedforrest/32/37502_2.png) [@jedforrest](https://discourse.julialang.org/u/jedforrest)\
**Post date:** [April 4, 2022, 6:43am UTC](https://discourse.julialang.org/t/custom-pretty-printing-post-processing/78844/3 "2022-04-04T06:43:48Z")

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`invoke` does look useful, but I don’t think it does exactly what I need here.

If I understand this function right, it is calling the `show` function for latex for the generic `Any` type. What I really need is the specific call for the type (in this case `::Vector{Equation}`), then do some extra changes before displaying the result.

In other words, I want to call the original version of the specialised `show` function, before it was overloaded by my new function. Is it possible to do that in Julia?

---

<div class="post-metadata">

**Author:** ![jedforrest](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jedforrest/32/37502_2.png) [@jedforrest](https://discourse.julialang.org/u/jedforrest)\
**Post date:** [April 5, 2022, 8:06am UTC](https://discourse.julialang.org/t/custom-pretty-printing-post-processing/78844/4 "2022-04-05T08:06:01Z")

</div>

I ended up fixing my problem in a much more straightforward way. Rather than getting the output of the previous method, I could just as easily call `latexify` again in my new function, like so:

```julia
using Latexify
function Base.show(io::IO, m::MIME"text/latex", x::MyType) 
    ltx = latexify(x)
    ltx = replace(ltx, "abc" => "def")
    print(io, ltx)
end

```

Turns out the simplest solution is sometimes the best!