clarify type reflection functions

- deprecate `fieldnames(v)` to disambiguate whether it operates on
  types or their instances.
- similarly, separate `nfields` for instances and `fieldcount` for types
- more errors for abstract types in `sizeof`
- allow `sizeof(::Symbol)`
- use more abstract type property access in src/

Author: JeffBezanson

NEWS.md

@@ -127,6 +127,12 @@ Deprecated or removed
     Ternaries must now include some amount of whitespace, e.g. `x ? a : b` rather than
     `x? a : b` ([#22523]).
 
+  * Calling `nfields` on a type to find out how many fields its instances have is deprecated.
+    Use `fieldcount` instead. Use `nfields` only to get the number of fields in a specific object ([#22350]).
+
+  * `fieldnames` now operates only on types. To get the names of fields in an object, use
+    `fieldnames(typeof(x))` ([#22350]).
+
 
 Julia v0.6.0 Release Notes
 ==========================

base/dates/io.jl

@@ -470,7 +470,7 @@ Parse a time from a time string `t` using a `DateFormat` object `df`.
 Time(t::AbstractString, df::DateFormat=ISOTimeFormat) = parse(Time, t, df)
 
 @generated function format(io::IO, dt::TimeType, fmt::DateFormat{<:Any,T}) where T
-    N = nfields(T)
+    N = fieldcount(T)
     quote
         ts = fmt.tokens
         loc = fmt.locale

base/deepcopy.jl

@@ -53,7 +53,7 @@ end
 
 function deepcopy_internal(x::ANY, stackdict::ObjectIdDict)
     T = typeof(x)::DataType
-    nf = nfields(T)
+    nf = nfields(x)
     (isbits(T) || nf == 0) && return x
     if haskey(stackdict, x)
         return stackdict[x]

base/deprecated.jl

@@ -1503,6 +1503,9 @@ function bkfact!(A::StridedMatrix, uplo::Symbol, symmetric::Bool = issymmetric(A
     return bkfact!(symmetric ? Symmetric(A, uplo) : Hermitian(A, uplo), rook)
 end
 
+@deprecate fieldnames(v) fieldnames(typeof(v))
+# nfields(::Type) deprecation in builtins.c: update nfields tfunc in inference.jl when it is removed.
+
 # END 0.7 deprecations
 
 # BEGIN 1.0 deprecations

base/distributed/messages.jl

@@ -95,7 +95,7 @@ let msg_cases = :(assert(false))
     for i = length(msgtypes):-1:1
         mti = msgtypes[i]
         msg_cases = :(if idx == $i
-                          return $(Expr(:call, QuoteNode(mti), fill(:(deserialize(s)), nfields(mti))...))
+                          return $(Expr(:call, QuoteNode(mti), fill(:(deserialize(s)), fieldcount(mti))...))
                       else
                           $msg_cases
                       end)

base/docs/Docs.jl

@@ -388,7 +388,7 @@ function summarize(io::IO, T::DataType, binding)
             " ", T, " <: ", supertype(T)
             )
     println(io, "```")
-    if !isempty(fieldnames(T))
+    if !T.abstract && T.name !== Tuple.name && !isempty(fieldnames(T))
         println(io, "**Fields:**")
         println(io, "```")
         pad = maximum(length(string(f)) for f in fieldnames(T))

base/docs/helpdb/Base.jl

@@ -1529,9 +1529,9 @@ Seek a stream to its beginning.
 seekstart
 
 """
-    nfields(x::DataType) -> Int
+    nfields(x) -> Int
 
-Get the number of fields of a `DataType`.
+Get the number of fields in the given object.
 """
 nfields
 

base/exports.jl

@@ -946,6 +946,7 @@ export
     fieldoffset,
     fieldname,
     fieldnames,
+    fieldcount,
     isleaftype,
     oftype,
     promote,

base/inference.jl

@@ -590,7 +590,7 @@ function isdefined_tfunc(args...)
             end
             if 1 <= idx <= a1.ninitialized
                 return Const(true)
-            elseif idx <= 0 || (idx > nfields(a1) && !isvatuple(a1))
+            elseif idx <= 0 || (!isvatuple(a1) && idx > fieldcount(a1))
                 return Const(false)
             end
         end
@@ -614,12 +614,14 @@ add_tfunc(Core.sizeof, 1, 1,
               x !== DataType && isleaftype(x) && return _const_sizeof(x)
               return Int
           end)
+old_nfields(x::ANY) = length((isa(x,DataType) ? x : typeof(x)).types)
 add_tfunc(nfields, 1, 1,
     function (x::ANY)
-        isa(x,Const) && return Const(nfields(x.val))
-        isa(x,Conditional) && return Const(nfields(Bool))
+        isa(x,Const) && return Const(old_nfields(x.val))
+        isa(x,Conditional) && return Const(old_nfields(Bool))
         if isType(x)
-            isleaftype(x.parameters[1]) && return Const(nfields(x.parameters[1]))
+            # TODO: remove with deprecation in builtins.c for nfields(::Type)
+            isleaftype(x.parameters[1]) && return Const(old_nfields(x.parameters[1]))
         elseif isa(x,DataType) && !x.abstract && !(x.name === Tuple.name && isvatuple(x)) && x !== DataType
             return Const(length(x.types))
         end
@@ -5545,8 +5547,8 @@ function is_allocation(e::ANY, sv::InferenceState)
         if isa(typ, DataType) && isleaftype(typ)
             nf = length(e.args) - 1
             names = fieldnames(typ)
-            @assert(nf <= nfields(typ))
-            if nf < nfields(typ)
+            @assert(nf <= length(names))
+            if nf < length(names)
                 # some fields were left undef
                 # we could potentially propagate Bottom
                 # for pointer fields

base/linalg/factorization.jl

@@ -49,9 +49,9 @@ end
 convert(::Type{Factorization{T}}, F::Factorization{T}) where {T} = F
 inv(F::Factorization{T}) where {T} = A_ldiv_B!(F, eye(T, size(F,1)))
 
-Base.hash(F::Factorization, h::UInt) = mapreduce(f -> hash(getfield(F, f)), hash, h, fieldnames(F))
-Base.:(==)(  F::T, G::T) where {T<:Factorization} = all(f -> getfield(F, f) == getfield(G, f), fieldnames(F))
-Base.isequal(F::T, G::T) where {T<:Factorization} = all(f -> isequal(getfield(F, f), getfield(G, f)), fieldnames(F))
+Base.hash(F::Factorization, h::UInt) = mapreduce(f -> hash(getfield(F, f)), hash, h, 1:nfields(F))
+Base.:(==)(  F::T, G::T) where {T<:Factorization} = all(f -> getfield(F, f) == getfield(G, f), 1:nfields(F))
+Base.isequal(F::T, G::T) where {T<:Factorization} = all(f -> isequal(getfield(F, f), getfield(G, f)), 1:nfields(F))
 
 # With a real lhs and complex rhs with the same precision, we can reinterpret
 # the complex rhs as a real rhs with twice the number of columns

base/options.jl

@@ -42,7 +42,7 @@ JLOptions() = unsafe_load(cglobal(:jl_options, JLOptions))
 
 function show(io::IO, opt::JLOptions)
     print(io, "JLOptions(")
-    fields = fieldnames(opt)
+    fields = fieldnames(JLOptions)
     nfields = length(fields)
     for (i, f) in enumerate(fields)
         v = getfield(opt, i)

base/reflection.jl

@@ -124,7 +124,8 @@ julia> fieldname(SparseMatrixCSC, 5)
 """
 fieldname(t::DataType, i::Integer) = t.name.names[i]::Symbol
 fieldname(t::UnionAll, i::Integer) = fieldname(unwrap_unionall(t), i)
-fieldname(t::Type{<:Tuple}, i::Integer) = i < 1 || i > nfields(t) ? throw(BoundsError(t, i)) : Int(i)
+fieldname(t::Type{<:Tuple}, i::Integer) =
+    i < 1 || i > fieldcount(t) ? throw(BoundsError(t, i)) : Int(i)
 
 """
     fieldnames(x::DataType)
@@ -139,16 +140,9 @@ julia> fieldnames(Hermitian)
  :uplo
 ```
 """
-function fieldnames(v)
-    t = typeof(v)
-    if !isa(t,DataType)
-        throw(ArgumentError("cannot call fieldnames() on a non-composite type"))
-    end
-    return fieldnames(t)
-end
-fieldnames(t::DataType) = Symbol[fieldname(t, n) for n in 1:nfields(t)]
+fieldnames(t::DataType) = Symbol[fieldname(t, n) for n in 1:fieldcount(t)]
 fieldnames(t::UnionAll) = fieldnames(unwrap_unionall(t))
-fieldnames(t::Type{<:Tuple}) = Int[n for n in 1:nfields(t)]
+fieldnames(t::Type{<:Tuple}) = Int[n for n in 1:fieldcount(t)]
 
 """
     Base.datatype_name(t) -> Symbol
@@ -265,7 +259,7 @@ false
 ```
 """
 isimmutable(x::ANY) = (@_pure_meta; (isa(x,Tuple) || !typeof(x).mutable))
-isstructtype(t::DataType) = (@_pure_meta; nfields(t) != 0 || (t.size==0 && !t.abstract))
+isstructtype(t::DataType) = (@_pure_meta; length(t.types) != 0 || (t.size==0 && !t.abstract))
 isstructtype(x) = (@_pure_meta; false)
 
 """
@@ -371,7 +365,7 @@ The byte offset of field `i` of a type relative to the data start. For example,
 use it in the following manner to summarize information about a struct:
 
 ```jldoctest
-julia> structinfo(T) = [(fieldoffset(T,i), fieldname(T,i), fieldtype(T,i)) for i = 1:nfields(T)];
+julia> structinfo(T) = [(fieldoffset(T,i), fieldname(T,i), fieldtype(T,i)) for i = 1:fieldcount(T)];
 
 julia> structinfo(Base.Filesystem.StatStruct)
 12-element Array{Tuple{UInt64,Symbol,DataType},1}:
@@ -440,6 +434,31 @@ end
 
 type_alignment(x::DataType) = (@_pure_meta; ccall(:jl_get_alignment, Csize_t, (Any,), x))
 
+"""
+    fieldcount(t::Type)
+
+Get the number of fields that an instance of the given type would have.
+An error is thrown if the type is too abstract to determine this.
+"""
+function fieldcount(t::ANY)
+    if t isa UnionAll || t isa Union
+        p = ccall(:jl_argument_datatype, Ptr{Void}, (Any,), t)
+        if p == C_NULL
+            error("type does not have a definite number of fields")
+        end
+        t = unsafe_pointer_to_objref(p)::DataType
+    elseif t == Union{}
+        return 0
+    end
+    if !(t isa DataType)
+        throw(TypeError(:fieldcount, "", Type, t))
+    end
+    if t.abstract || (t.name === Tuple.name && isvatuple(t))
+        error("type does not have a definite number of fields")
+    end
+    return length(t.types)
+end
+
 # return all instances, for types that can be enumerated
 
 """

base/replutil.jl

@@ -153,7 +153,7 @@ end
 
 function show(io::IO, ::MIME"text/plain", opt::JLOptions)
     println(io, "JLOptions(")
-    fields = fieldnames(opt)
+    fields = fieldnames(JLOptions)
     nfields = length(fields)
     for (i, f) in enumerate(fields)
         v = getfield(opt, i)

base/serialize.jl

@@ -605,7 +605,7 @@ function serialize_any(s::AbstractSerializer, x::ANY)
         return write_as_tag(s.io, tag)
     end
     t = typeof(x)::DataType
-    nf = nfields(t)
+    nf = nfields(x)
     if nf == 0 && t.size > 0
         serialize_type(s, t)
         write(s.io, x)
@@ -718,7 +718,7 @@ function handle_deserialize(s::AbstractSerializer, b::Int32)
         return deserialize_symbol(s, Int(read(s.io, Int32)::Int32))
     end
     t = desertag(b)
-    if t.mutable && nfields(t) > 0
+    if t.mutable && length(t.types) > 0  # manual specialization of fieldcount
         slot = s.counter; s.counter += 1
         push!(s.pending_refs, slot)
     end
@@ -1046,7 +1046,7 @@ end
 
 # default DataType deserializer
 function deserialize(s::AbstractSerializer, t::DataType)
-    nf = nfields(t)
+    nf = length(t.types)
     if nf == 0 && t.size > 0
         # bits type
         return read(s.io, t)

base/show.jl

@@ -127,7 +127,7 @@ function show_default(io::IO, x::ANY)
     t = typeof(x)::DataType
     show(io, t)
     print(io, '(')
-    nf = nfields(t)
+    nf = nfields(x)
     nb = sizeof(x)
     if nf != 0 || nb == 0
         if !show_circular(io, x)
@@ -1186,7 +1186,7 @@ function dump(io::IO, x::ANY, n::Int, indent)
     else
         print(io, T)
     end
-    if nfields(T) > 0
+    if nfields(x) > 0
         if n > 0
             for field in (isa(x,Tuple) ? (1:length(x)) : fieldnames(T))
                 println(io)
@@ -1259,12 +1259,14 @@ function dump(io::IO, x::DataType, n::Int, indent)
                 tvar_io = IOContext(tvar_io, :unionall_env => tparam)
             end
         end
-        fields = fieldnames(x)
-        fieldtypes = x.types
-        for idx in 1:length(fields)
-            println(io)
-            print(io, indent, "  ", fields[idx], "::")
-            print(tvar_io, fieldtypes[idx])
+        if !x.abstract
+            fields = fieldnames(x)
+            fieldtypes = x.types
+            for idx in 1:length(fields)
+                println(io)
+                print(io, indent, "  ", fields[idx], "::")
+                print(tvar_io, fieldtypes[idx])
+            end
         end
     end
     nothing

base/summarysize.jl

@@ -8,6 +8,7 @@ struct SummarySize
     chargeall::Any
 end
 
+_nfields(x::ANY) = length(typeof(x).types)
 
 """
     Base.summarysize(obj; exclude=Union{...}, chargeall=Union{...}) -> Int
@@ -41,7 +42,7 @@ function summarysize(obj::ANY;
                 val = x[i]
             end
         else
-            nf = nfields(x)
+            nf = _nfields(x)
             ft = typeof(x).types
             if !isbits(ft[i]) && isdefined(x, i)
                 val = getfield(x, i)
@@ -65,11 +66,11 @@ end
 @noinline function _summarysize(ss::SummarySize, obj::ANY)
     key = pointer_from_objref(obj)
     haskey(ss.seen, key) ? (return 0) : (ss.seen[key] = true)
-    if nfields(obj) > 0
+    if _nfields(obj) > 0
         push!(ss.frontier_x, obj)
         push!(ss.frontier_i, 1)
     end
-    if isa(obj, UnionAll)
+    if isa(obj, UnionAll) || isa(obj, Union)
         # black-list of items that don't have a Core.sizeof
         return 2 * sizeof(Int)
     end
@@ -84,7 +85,7 @@ function (ss::SummarySize)(obj::DataType)
     key = pointer_from_objref(obj)
     haskey(ss.seen, key) ? (return 0) : (ss.seen[key] = true)
     size::Int = 7 * Core.sizeof(Int) + 6 * Core.sizeof(Int32)
-    size += 4 * nfields(obj) + ifelse(Sys.WORD_SIZE == 64, 4, 0)
+    size += 4 * _nfields(obj) + ifelse(Sys.WORD_SIZE == 64, 4, 0)
     size += ss(obj.parameters)::Int
     size += ss(obj.types)::Int
     return size

doc/src/manual/types.md

@@ -352,7 +352,7 @@ Stacktrace:
 You may find a list of field names using the `fieldnames` function.
 
 ```jldoctest footype
-julia> fieldnames(foo)
+julia> fieldnames(Foo)
 3-element Array{Symbol,1}:
  :bar
  :baz

doc/src/stdlib/base.md

@@ -257,6 +257,7 @@ Base.names
 Core.nfields
 Base.fieldnames
 Base.fieldname
+Base.fieldcount
 Base.datatype_module
 Base.datatype_name
 Base.isconst