Reflection and introspection

    The exported names for a are available using names(m::Module), which will return an array of 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 . For example, given the following type, fieldnames(Point) returns a tuple of Symbols representing the field names:

    The type of each field in a Point object is stored in the types field of the Point variable itself:

    1. julia> Point.types
    2. 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:

    The direct subtypes of any DataType may be listed using . For example, the abstract DataType AbstractFloat has four (concrete) subtypes:

    1. julia> subtypes(AbstractFloat)
    2. 4-element Array{Any,1}:
    3.  BigFloat
    4.  Float16
    5.  Float64

    Any abstract subtype will also be included in this list, but further subtypes thereof will not; recursive application of 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. 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.

    The methods of any generic function may be listed using . 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 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:

    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:

    1. julia> Meta.lower(@__MODULE__, :([1+2, sin(0.5)]) )
    2. :($(Expr(:thunk, CodeInfo(
    3.  1 ─ %1 = 1 + 2
    4.  │   %2 = sin(0.5)
    5.  │   %3 = (Base.vect)(%1, %2)
    6. ))))

    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 , and the type-inferred form is available using code_typed. adds highlighting to the output of code_typed.

    Closer to the machine, the LLVM intermediate representation of a function may be printed using by , 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:

    See , @code_typed, , @code_llvm, and .