Printing in OSCAR

The following dection contains more details and examples on how to implement OSCAR's 2+1 printing modes. The specifications and a minimal example may be found in the Developer Style Guide.

Implementing show functions

Here is the translation between :detail, one line and terse, where io is an IO object (such as stdout or an IOBuffer):

show(io, MIME"text/plain"(), x)                # detailed printing
print(io, x)                                   # one line printing
print(terse(io), x)                            # terse printing

For reference, string interpolation "$(x)" uses one line printing via print(io, x), while on the REPL detailed printing is used to show top level objects.

Mockup

Detailed printing with a new line

struct NewRing
  base_ring
end

base_ring(R::NewRing) = R.base_ring

The following is a template for detailed printing. Note that at least one new line is needed for technical reasons. see below why.

function Base.show(io::IO, ::MIME"text/plain", R::NewRing)
  println(io, "I am a new ring")  # at least one new line is needed
  println(io, "I print with newlines")
  print(io, base_ring(R)) # the last print statement must not add a new line
end

The following is a template for one line and terse printing.

function Base.show(io::IO, R::NewRing)
  if is_terse(io)
    # no nested printing
    print(io, "terse printing of newring ")
  else
    # nested printing allowed, preferably terse
    print(io, "one line printing of newring with ")
    print(terse(io), "terse ", base_ring(R))
  end
end

And this is how it looks like:

julia> R = NewRing(QQ)
I am a new ring
I print with newlines
QQ

julia> [R,R]
2-element Vector{NewRing}:
 one line printing of newring with terse QQ
 one line printing of newring with terse QQ

Detailed printing in a single line

This version needs to be used in case the detailed printing does not contain newlines. Then detailed and one line printing agree. The if clause takes care of terse printing as well.

struct NewRing2
  base_ring
end

base_ring(R::NewRing2) = R.base_ring

function Base.show(io::IO, R::NewRing2)
  if is_terse(io)
    # no nested printing
    print(io, "terse printing of newring")
  else
    # nested printing allowed, preferably terse
    print(io, "I am a new ring and always print in one line " )
    print(terse(io), base_ring(R))
  end
end

And this is how it looks like:

julia> R = NewRing2(QQ)
I am a new ring and always print in one line QQ

julia> [R,R]
2-element Vector{NewRing2}:
 I am a new ring and always print in one line Rational Field
 I am a new ring and always print in one line Rational Field

julia> print(terse(Base.stdout) ,R)
terse printing of newring

The terse printing uses an IOContext (see IOContext from the Julia documentation) to pass information to other show methods invoked recursively (for example in nested printings). The same mechanism can be used to pass other context data. For instance, this is used by the Scheme code in some nested printings which invoke several objects whose printing depends on a given covering: we use IOContext to pass a fix covering to the printing of each sub-object for consistency and readability.

The following is not working as expected and should not be used

This example does not work correctly because the detailed printing does not include a newline, which is expected by the Julia printing system. To correctly support single line detailed printing, read the preceding section.

function Base.show(io::IO, ::MIME"text/plain", R::NewRing)  # do not implement me like this
  print(io, "I am a new ring with a detailed printing of one line")
end

Then the following will not be used for array/tuple printing. It will be used for print(io, R::NewRing) though.

function Base.show(io::IO, R::NewRing)
  if is_terse(io)
    print(io, "terse printing of newring")
  else # this is what we call one line
    print(io, "one line printing of newring with ")
    print(terse(io), "terse ", R.base_ring)
  end
end

This example illustrates the unexpected behavior.

julia> R = NewRing(1)

julia> R
I am a new ring with a detailed printing of one line

julia> [R,R]  # one line printing is ignored
2-element Vector{NewRing}:
 I am a new ring with a detailed printing of one line
 I am a new ring with a detailed printing of one line

julia> print(Base.stdout, R)
one line printing of newring with terse QQ

Advanced printing functionality

To facilitate printing of nested mathematical structures, we provide a modified IOCustom object. To create one, we use the following command:

pretty(io::IO) -> IOCustom

julia> io = AbstractAlgebra.pretty(stdout);

The IOCustom object allows one to locally control:

• indentation using Indent() and Dedent(),
• capitalization using Lowercase() and LowercaseOff().

Example

We illustrate this with an example

struct A{T}
  x::T
end

function Base.show(io::IO, a::A)
  io = pretty(io)
  println(io, "Something of type A")
  print(io, Indent(), "over ", Lowercase(), a.x)
  print(io, Dedent()) # don't forget to undo the indentation!
end

struct B
end

function Base.show(io::IO, b::B)
  io = pretty(io)
  print(io, LowercaseOff(), "Hilbert thing")
end

At the REPL, this will then be printed as follows:

julia> A(2)
Something of type A
  over 2

julia> A(A(2))
Something of type A
  over something of type A
    over 2

julia> A(B())
Something of type A
  over Hilbert thing

Moreover, one can control the pluralization of nouns when printing a set of elements with a variable number of objects. For this, one can use ItemQuantity:

Example

We illustrate this with an example

julia> struct C{T}
       x::Vector{T}
       end

julia> function Base.show(io::IO, c::C{T}) where T
       x = c.x
       n = length(x)
       print(io, "Something with ", ItemQuantity(n, "element"), " of type $T")
       end

At the REPL, this will then be printed as follows:

julia> C(Int[2,3,4])
Something with 3 elements of type Int64

julia> C(Int[])
Something with 0 elements of type Int64

julia> C(Int[6])
Something with 1 element of type Int64

LaTeX and Unicode printing

LaTeX output

Some types support LaTeX output.

julia> Qx, x = QQ[:x];

julia> show(stdout, "text/latex", x^2 + 2x + x^10)
x^{10} + x^{2} + 2 x

julia> show(stdout, "text/latex", Qx[x x^2; 1 1])
\begin{array}{cc}
x & x^{2} \\
1 & 1
\end{array}

Base.show(io::IOContext, ::MIME"text/latex")

Unicode printing

Per default output should be ASCII only (no Unicode). Implementors of Base.show and related functions can branch on the output of Oscar.is_unicode_allowed() to display objects using non-ASCII characters. This will then be used for users which enabled Unicode using allow_unicode(true). Note that

• there must be a default ASCII only output, since this is the default setting for new users, and
• OSCAR library code is not allowed to call Oscar.allow_unicode.

Objects may follow the value of Oscar.is_unicode_allowed() at the time of their creation for their printing, i.e. ignore later changes of the setting. This is useful for objects storing a string representation of themselves, e.g. generators of a module.

Here is an example with and without output using Unicode:

  struct AtoB
  end

  function Base.show(io::IO, ::AtoB)
    if Oscar.is_unicode_allowed()
      print(io, "A→B")
    else
      print(io, "A->B")
    end
  end

On using @show_name, @show_special, @show_special_elem

• All show methods for parent objects such as rings or modules should use the @show_name macro. This macro ensures that if the object has a name (including one derived from the name of a Julia REPL variable to which the object is currently assigned) then in a compact or terse io context it is printed using that name. Here is an example illustrating this:

  julia> vector_space(GF(2), 2)
  Vector space of dimension 2 over prime field of characteristic 2
  
  julia> K = GF(2)
  Finite field F_2
  
  julia> vector_space(K, 2)
  Vector space of dimension 2 over K

  The documentation for AbstractAlgebra.get_name describes how the name is determined.

• All show methods for parent objects should also use @show_special. This checks if an attribute :show is present. If so, it has to be a function taking IO, optionally a MIME-type, and the object. This is then called instead of the usual show function.

• Similarly, all show methods for element objects may use @show_special_elem which checks if an attribute :show_elem is present in the object's parent. The semantics are the same as for @show_special.

For details please consult the Advanced printing section of the AbstractAlgebra documentation.