Expr to String conversion


#1

Is there a function that converts an expression or symbol to a String representation?


#2

repr


#3

repr in julia does not promise to do this.
I think it should.
But it does not.

help?> repr
repr(x)
Create a string from any value using the showall function.

help?> showall
showall(x)
Similar to show, except shows all elements of arrays.

help?> show
show(stream, mime, x)
The display functions ultimately call show in order to write an object x as a given mime type to a given I/O stream (usually a memory buffer), if possible. In order to
provide a rich multimedia representation of a user-defined type T, it is only necessary to define a new show method f
…
The default MIME type is MIME"text/plain". There is a fallback definition for text/plain output that calls show with 2 arguments. Therefore, this case should be
handled by defining a 2-argument show(stream::IO, x::MyType) method.
…
show(x)

Write an informative text representation of a value to the current output stream. New types should overload show(io, x) where the first argument is a stream. The
representation used by show generally includes Julia-specific formatting and type information.

By docs,
we have an indirect statement that
the results of repr will be similar to those of a function that says that it will return a “representation … [that] …generally includes Julia-specific formatting and type information.”

Which is a pretty weak promise, vs: eval(parse(repr(x)) == x
Which would be a nicely strong statement.
But has its own issues like requiring equality to be defined for all types with repr defined.
And its own issues for objects of types that can not be trivially duplicated (eg file handles).

A weaker statement might be something like:
x->eval(parse(repr(x))) shall be functionally equivalent to deepcopy.

Basically as far as julia’s docs for repr are concerned,
repr probably gives you an in informative string.
By a stretch of meaning one could interpret “julia specific formatting” as “parsable”.
In practice it tends to follow the promise I suggested.

Contrast Python :

repr(object)
Return a string containing a printable representation of an object. For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a repr() method.

Very clear as to what it does. Does have some wiggle with saying “for many types”,
but for types following the convention then they either return an eval able string that gives a equal object,
or if they can not, then the string they give back is wrapped in angle brrackeds and contains the type name and some additional information.