Add hasfield to check for field existence

NEWS.md

@@ -1269,6 +1269,8 @@ Deprecated or removed
 
  * `atan2` is now a 2-argument method of `atan` ([#27248]).
 
+ * `hasfield` was added to check if a composite type has a field of a given name ([#27582])
+
 Command-line option changes
 ---------------------------
 

base/exports.jl

@@ -714,6 +714,7 @@ export
  fieldname,
  fieldnames,
  fieldcount,
+ hasfield,
  propertynames,
  isabstracttype,
  isbitstype,

base/reflection.jl

@@ -560,6 +560,27 @@ function fieldindex(T::DataType, name::Symbol, err::Bool=true)
  return Int(ccall(:jl_field_index, Cint, (Any, Any, Cint), T, name, err)+1)
 end
 
+"""
+ hasfield(T, name::Symbol)
+
+Return `true` if a composite DataType `T` has a field called `name`.
+
+# Examples
+```jldoctest
+julia> struct Foo
+ x::Int64
+ y::String
+ end
+
+julia> Base.hasfield(Foo, :x)
+true
+
+julia> Base.hasfield(Foo, :balloon)
+false
+```
+"""
+hasfield(T::DataType, name::Symbol) = fieldindex(T, name, false) > 0
+
 argument_datatype(@nospecialize t) = ccall(:jl_argument_datatype, Any, (Any,), t)
 function argument_mt(@nospecialize t)
  dt = argument_datatype(t)

doc/src/base/base.md

@@ -154,6 +154,7 @@ Base.isbitstype
 Core.fieldtype
 Base.fieldcount
 Base.fieldoffset
+Base.hasfield
 Base.datatype_alignment
 Base.datatype_haspadding
 Base.datatype_pointerfree

test/reflection.jl

@@ -230,6 +230,11 @@ tlayout = TLayout(5,7,11)
 @test_throws ArgumentError fieldname(TLayout, 4)
 @test_throws BoundsError fieldoffset(TLayout, 4)
 
+for name in (:x, :y, :z)
+ @test hasfield(TLayout, name)
+end
+@test !hasfield(TLayout, :parameters)
+
 @test fieldtype(Tuple{Vararg{Int8}}, 1) === Int8
 @test fieldtype(Tuple{Vararg{Int8}}, 10) === Int8
 @test_throws BoundsError fieldtype(Tuple{Vararg{Int8}}, 0)