Reflection and introspection
Julia provides a variety of runtime reflection capabilities.
Module bindings
The exported names for a Module
are available using names(m::Module)
, which will return an array of Symbol
elements representing the exported bindings. names(m::Module, all = true)
returns symbols for all bindings in m
, regardless of export status.
DataType fields
The names of DataType
fields may be interrogated using fieldnames
. For example, given the following type, fieldnames(Point)
returns a tuple of Symbol
s representing the field names:
julia> struct Point
x::Int
y
end
julia> fieldnames(Point)
(:x, :y)
The type of each field in a Point
object is stored in the types
field of the Point
variable itself:
julia> Point.types
svec(Int64, Any)
While x
is annotated as an Int
, y
was unannotated in the type definition, therefore y
defaults to the Any
type.
Types are themselves represented as a structure called DataType
:
julia> typeof(Point)
DataType
Note that fieldnames(DataType)
gives the names for each field of DataType
itself, and one of these fields is the types
field observed in the example above.
Subtypes
The direct subtypes of any DataType
may be listed using subtypes
. For example, the abstract DataType
AbstractFloat
has four (concrete) subtypes:
julia> subtypes(AbstractFloat)
4-element Vector{Any}:
BigFloat
Float16
Float32
Float64
Any abstract subtype will also be included in this list, but further subtypes thereof will not; recursive application of subtypes
may be used to inspect the full type tree.
DataType layout
The internal representation of a DataType
is critically important when interfacing with C code and several functions are available to inspect these details. isbits(T::DataType)
returns true if T
is stored with C-compatible alignment. fieldoffset(T::DataType, i::Integer)
returns the (byte) offset for field i relative to the start of the type.
Function methods
The methods of any generic function may be listed using methods
. The method dispatch table may be searched for methods accepting a given type using methodswith
.
Expansion and lowering
As discussed in the Metaprogramming section, the macroexpand
function gives the unquoted and interpolated expression (Expr
) form for a given macro. To use macroexpand
, quote
the expression block itself (otherwise, the macro will be evaluated and the result will be passed instead!). For example:
julia> macroexpand(@__MODULE__, :(@edit println("")) )
:(InteractiveUtils.edit(println, (Base.typesof)("")))
The functions Base.Meta.show_sexpr
and dump
are used to display S-expr style views and depth-nested detail views for any expression.
Finally, the Meta.lower
function gives the lowered
form of any expression and is of particular interest for understanding how language constructs map to primitive operations such as assignments, branches, and calls:
julia> Meta.lower(@__MODULE__, :( [1+2, sin(0.5)] ))
:($(Expr(:thunk, CodeInfo(
@ none within `top-level scope`
1 ─ %1 = 1 + 2
│ %2 = sin(0.5)
│ %3 = Base.vect(%1, %2)
└── return %3
))))
Intermediate and compiled representations
Inspecting the lowered form for functions requires selection of the specific method to display, because generic functions may have many methods with different type signatures. For this purpose, method-specific code-lowering is available using code_lowered
, and the type-inferred form is available using code_typed
. code_warntype
adds highlighting to the output of code_typed
.
Closer to the machine, the LLVM intermediate representation of a function may be printed using by code_llvm
, and finally the compiled machine code is available using code_native
(this will trigger JIT compilation/code generation for any function which has not previously been called).
For convenience, there are macro versions of the above functions which take standard function calls and expand argument types automatically:
julia> @code_llvm +(1,1)
define i64 @"julia_+_130862"(i64, i64) {
top:
%2 = add i64 %1, %0
ret i64 %2
}
For more informations see @code_lowered
, @code_typed
, @code_warntype
, @code_llvm
, and @code_native
.
Printing of debug information
The aforementioned functions and macros take the keyword argument debuginfo
that controls the level debug information printed.
julia> @code_typed debuginfo=:source +(1,1)
CodeInfo(
@ int.jl:53 within `+'
1 ─ %1 = Base.add_int(x, y)::Int64
└── return %1
) => Int64
Possible values for debuginfo
are: :none
, :source
, and:default
. Per default debug information is not printed, but that can be changed by setting Base.IRShow.default_debuginfo[] = :source
.