# Overriding Base.show for Int64 causes madness

**URL:** <https://discourse.julialang.org/t/overriding-base-show-for-int64-causes-madness/63961>\
**Category:** General Usage\
**Created:** [July 2, 2021, 11:06pm UTC](https://discourse.julialang.org/t/overriding-base-show-for-int64-causes-madness/63961 "2021-07-02T23:06:21Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![ess3sq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ess3sq/32/26834_2.png) [@ess3sq](https://discourse.julialang.org/u/ess3sq)\
**Post date:** [July 2, 2021, 11:06pm UTC](https://discourse.julialang.org/t/overriding-base-show-for-int64-causes-madness/63961/1 "2021-07-02T23:06:21Z")

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Dear all,

I am dumbstruck. I just tried the pointless endeavour overriding `Base.show` for `Int64`. This is what I did (REPL):

```julia
julia> import Base.show
julia> Base.show(io::IO, x::Int64) = println("Hello. I am $x")
julia> q = 1

```

The output is:

```julia
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2237 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2419
display_error at ./client.jl:107
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2237 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2419
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1703 [inlined]
jl_f__call_latest at /buildworker/worker/package_linux64/build/src/builtins.c:714
#invokelatest#2 at ./essentials.jl:708 [inlined]
invokelatest at ./essentials.jl:706 [inlined]
_start at ./client.jl:487
jfptr__start_34289.clone_1 at /home/lorenzo/bin/julia-1.6.1/lib/julia/sys.so (unknown line)
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2237 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2419
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1703 [inlined]
true_main at /buildworker/worker/package_linux64/build/src/jlapi.c:560
repl_entrypoint at /buildworker/worker/package_linux64/build/src/jlapi.c:702
main at julia (unknown line)
__libc_start_main at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
unknown function (ip: 0x4007d8)

```

repeated thousands of times, during which I am essentially locked out of the terminal.  
So the question, of course, is… what gives?

Now I am guessing this is due to the fact that `"Hello. I am $x"` calls `Base.show` in an infinitely recursive manner. What I don’t understand, however, is the output dump. Is that some sort of stacktrace due to stack smashing? It is worth noting that Julia does something for a while (silently) before dumping that garbage. Also, if that is the case, is that Julia’s normal wy of handling infinite recursion?

Any hints/confirmations would be appreciated! PS. I am using Julia 1.6.1.

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<div class="post-metadata">

**Author:** ![ctkelley](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ctkelley/32/10684_2.png) [@ctkelley](https://discourse.julialang.org/u/ctkelley)\
**Post date:** [July 2, 2021, 11:40pm UTC](https://discourse.julialang.org/t/overriding-base-show-for-int64-causes-madness/63961/2 "2021-07-02T23:40:19Z")

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I do something like this for floats in my startup.jl file. I use @printf rather than println. Here’s an exampel

```julia
using Printf 
Base.show(io::IO, f::Float64) = @printf(io, "%1.5e", f)

```

The idea is that I only want five figures in the mantissa to show up on the screen.

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**Author:** ![mbaz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbaz/32/17295_2.png) [@mbaz](https://discourse.julialang.org/u/mbaz)\
**Post date:** [July 2, 2021, 11:54pm UTC](https://discourse.julialang.org/t/overriding-base-show-for-int64-causes-madness/63961/3 "2021-07-02T23:54:04Z")

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I suspect that interpolating an integer calls `show` and that is causing an infinite recursion. It’s not the whole story, though:

```julia
julia> Base.show(io::IO, x::Int64) = println("hello")

julia> q=1hello
hello
jq=1a>
hello
hello

hello
hello
julia>

```

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<div class="post-metadata">

**Author:** ![ctkelley](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ctkelley/32/10684_2.png) [@ctkelley](https://discourse.julialang.org/u/ctkelley)\
**Post date:** [July 3, 2021, 12:05am UTC](https://discourse.julialang.org/t/overriding-base-show-for-int64-causes-madness/63961/4 "2021-07-03T00:05:03Z")

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Just tried this and it seems to work

```julia
julia> Base.show(io::IO, f::Float64) = @printf(io, "%s %1.5e", "Hello ", f)

julia> sqrt(2)
Hello 1.41421e+00

julia> exp(1)
Hello 2.71828e+00

```

So there does not seem to be a problem with Floats. With Ints I’m seeing variations on the insanity that @mbaz reports

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<div class="post-metadata">

**Author:** ![ess3sq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ess3sq/32/26834_2.png) [@ess3sq](https://discourse.julialang.org/u/ess3sq)\
**Post date:** [July 3, 2021, 10:43am UTC](https://discourse.julialang.org/t/overriding-base-show-for-int64-causes-madness/63961/5 "2021-07-03T10:43:41Z")

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That’s interesting. Could it be that `@printf` does not use `Base.show` but it’s own implementation for converting numbers to strings based on the format specifiers?

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**Author:** ![simeonschaub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/simeonschaub/32/216566_2.png) [@simeonschaub](https://discourse.julialang.org/u/simeonschaub)\
**Post date:** [July 3, 2021, 12:09pm UTC](https://discourse.julialang.org/t/overriding-base-show-for-int64-causes-madness/63961/6 "2021-07-03T12:09:38Z")

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Just to emphasize: This is [type piracy](https://docs.julialang.org/en/v1/manual/style-guide/#Avoid-type-piracy) and an especially dangerous one since this method is used in a lot of other places. It is expected that all kind of things can break if you redefine this.

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<div class="post-metadata">

**Author:** ![ess3sq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ess3sq/32/26834_2.png) [@ess3sq](https://discourse.julialang.org/u/ess3sq)\
**Post date:** [July 3, 2021, 10:32pm UTC](https://discourse.julialang.org/t/overriding-base-show-for-int64-causes-madness/63961/7 "2021-07-03T22:32:40Z")

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I have no doubts about this being a ridiculously dumb idea that would probably not be any smarter than overriding arithmetic operators for Int64’s. However, the point I wanted to discuss is the direct result, not the catastrophic implications.