Convert to string without losing the colon character for Symbol variables

General Usage
,
1

If I convert a Symbol to a string, the colon that is used to signify that it is a symbol is lost. For example,

julia> a = :abc
:abc

julia> string(a)    # String does not have a colon
"abc"

I know I can create a very simple function to preserve the colon for Symbols but not alter the string formed for other variables but I was wondering if there is already an existing function for this.

2

The colon is not actually part of the Symbol object, which is why you don’t get the colon when you create a String from it. Similar to how Vector is not part of the vector object itself when printing an array.

You can do ':' * string(a) to prepend the string with a colon.

3

There is repr:

julia> repr(:foo)
":foo"
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4

It isnot a string but show(a) looks like you want

5

That’s not guaranteed to give you the colon, because the colon is not part of the Symbol:

julia> Symbol("1")
Symbol("1")

julia> repr(Symbol("1"))
"Symbol(\"1\")"
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6

To expand a bit on what I mean when I say “the colon is not part of the symbol”; contrary to popular belief, the prefixed colon is not the “make this a symbol” operator:

julia> a = :1
1

julia> a isa Symbol
false

julia> typeof(a)
Int64

Used like this, the colon is a quasi-quote operator. This gets a bit technical, but this more or less means “give me the next fully-formed expression as the parser understands it”. For numbers, that means giving back a numeric literal, for some text that means a symbol and so on.

The issue is the following - Symbol("foo bar") is a perfectly valid symbol! You can’t produce it with :foo bar though, because the first fully formed expression after : is just foo:

julia> :foo bar
ERROR: ParseError:
# Error @ REPL[6]:1:5
:foo bar
#   └──┘ ── extra tokens after end of expression
Stacktrace:
 [1] top-level scope
   @ none:1

(thanks to @c42f for the amazing PR on the most recent master branch! It’s very useful to explain things like this).

Similarly, Symbol("1") is a perfectly valid symbol, yet you can’t get it by writing :1 because that returns an integer literal, as shown above.

So while I don’t know why you want the prefixed colon, note that it’s not part of the symbol; I’m not sure prefixing it in strings you create from a symbol is going to achieve what you’re ultimately trying to do. While repr doesn’t always give a leading : (and it can’t), it’s likely going to be what you’re looking for anyway.

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7

I want to convert to a string so unfortunately show() does not work. repr() looks usable.

8

@Sukera: Thanks for the expanded reply. My usage is to print Dict key, value info where the key can be a Symbol like :orientation . I want the output to include the colon to alert the user that the key is a Symbol. There will not be any keys like Symbol("1") so indeed, for my purposes, repr() looks to be what I need.

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9

under the hood, repr calls show

julia> a=:foo
:foo

julia> sprint(show,a)
":foo"

julia> typeof(sprint(show,a))
String
10

@Sukera, did you mean this PR?

11

@rocco_sprmnt21: Interesting! If I try to use string instead of sprint I get:

julia> string(show(a))
:foo"nothing"

I do not understand this output. Can you explain this? Thanks.

12

As far as I understand, the show(something) function sends to the stdout (the screen) ‘something’ but returns nothing.

julia> show(a)
:foo

Introducing this as an argument to the string function produces the output of the show() function, because the argument is executed, and transforms the returned value into a string, ie “nothing”.

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13

No, I was referring to this one, which produced the fancy error message in the syntax error, showing that :foo is already a fully formed expression.

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