Scoped values (#50958)

ScopedVariables are containers whose observed value depends the current
dynamic scope. This implementation is inspired by
https://openjdk.org/jeps/446

A scope is introduced with the `scoped` function that takes a lambda to
execute within the new scope. The value of a `ScopedValue` is
constant within that scope and can only be set upon introduction
of a new scope.

Scopes are propagated across tasks boundaries.

Storage is implemented using a persistent dictionary.

NEWS.md

@@ -3,6 +3,7 @@ Julia v1.11 Release Notes
 
 New language features
 ---------------------
+* `ScopedValue` implement dynamic scope with inheritance across tasks ([#50958]).
 
 Language changes
 ----------------

base/Base.jl

@@ -330,10 +330,6 @@ using .Libc: getpid, gethostname, time, memcpy, memset, memmove, memcmp
 const libblas_name = "libblastrampoline" * (Sys.iswindows() ? "-5" : "")
 const liblapack_name = libblas_name
 
-# Logging
-include("logging.jl")
-using .CoreLogging
-
 # Concurrency (part 2)
 # Note that `atomics.jl` here should be deprecated
 Core.eval(Threads, :(include("atomics.jl")))
@@ -343,6 +339,14 @@ include("task.jl")
 include("threads_overloads.jl")
 include("weakkeydict.jl")
 
+# ScopedValues
+include("scopedvalues.jl")
+using .ScopedValues
+
+# Logging
+include("logging.jl")
+using .CoreLogging
+
 include("env.jl")
 
 # functions defined in Random

base/boot.jl

@@ -163,7 +163,7 @@
 #    result::Any
 #    exception::Any
 #    backtrace::Any
-#    logstate::Any
+#    scope::Any
 #    code::Any
 #end
 

base/exports.jl

@@ -648,6 +648,11 @@ export
     sprint,
     summary,
 
+# ScopedValue
+    with,
+    @with,
+    ScopedValue,
+
 # logging
     @debug,
     @info,

base/logging.jl

@@ -492,31 +492,23 @@ end
 
 LogState(logger) = LogState(LogLevel(_invoked_min_enabled_level(logger)), logger)
 
+const CURRENT_LOGSTATE = ScopedValue{LogState}()
+
 function current_logstate()
-    logstate = current_task().logstate
-    return (logstate !== nothing ? logstate : _global_logstate)::LogState
+    maybe = @inline Base.ScopedValues.get(CURRENT_LOGSTATE)
+    return something(maybe, _global_logstate)::LogState
 end
 
 # helper function to get the current logger, if enabled for the specified message type
 @noinline Base.@constprop :none function current_logger_for_env(std_level::LogLevel, group, _module)
-    logstate = current_logstate()
+    logstate = @inline current_logstate()
     if std_level >= logstate.min_enabled_level || env_override_minlevel(group, _module)
         return logstate.logger
     end
     return nothing
 end
 
-function with_logstate(f::Function, logstate)
-    @nospecialize
-    t = current_task()
-    old = t.logstate
-    try
-        t.logstate = logstate
-        f()
-    finally
-        t.logstate = old
-    end
-end
+with_logstate(f::Function, logstate) = @with(CURRENT_LOGSTATE => logstate, f())
 
 #-------------------------------------------------------------------------------
 # Control of the current logger and early log filtering

base/scopedvalues.jl

@@ -0,0 +1,199 @@
+# This file is a part of Julia. License is MIT: https://julialang.org/license
+
+module ScopedValues
+
+export ScopedValue, with, @with
+
+"""
+    ScopedValue(x)
+
+Create a container that propagates values across dynamic scopes.
+Use [`with`](@ref) to create and enter a new dynamic scope.
+
+Values can only be set when entering a new dynamic scope,
+and the value referred to will be constant during the
+execution of a dynamic scope.
+
+Dynamic scopes are propagated across tasks.
+
+# Examples
+
+```jldoctest
+julia> const sval = ScopedValue(1);
+
+julia> sval[]
+1
+
+julia> with(sval => 2) do
+           sval[]
+       end
+2
+
+julia> sval[]
+1
+```
+
+!!! compat "Julia 1.11"
+    Scoped values were introduced in Julia 1.11. In Julia 1.8+ a compatible
+    implementation is available from the package ScopedValues.jl.
+"""
+mutable struct ScopedValue{T}
+    const has_default::Bool
+    const default::T
+    ScopedValue{T}() where T = new(false)
+    ScopedValue{T}(val) where T = new{T}(true, val)
+    ScopedValue(val::T) where T = new{T}(true, val)
+end
+
+Base.eltype(::ScopedValue{T}) where {T} = T
+
+"""
+    isassigned(val::ScopedValue)
+
+Test if the ScopedValue has a default value.
+"""
+Base.isassigned(val::ScopedValue) = val.has_default
+
+const ScopeStorage = Base.PersistentDict{ScopedValue, Any}
+
+mutable struct Scope
+    values::ScopeStorage
+end
+
+function Scope(parent::Union{Nothing, Scope}, key::ScopedValue{T}, value) where T
+    val = convert(T, value)
+    if parent === nothing
+        return Scope(ScopeStorage(key=>val))
+    end
+    return Scope(ScopeStorage(parent.values, key=>val))
+end
+
+function Scope(scope, pairs::Pair{<:ScopedValue}...)
+    for pair in pairs
+        scope = Scope(scope, pair...)
+    end
+    return scope::Scope
+end
+Scope(::Nothing) = nothing
+
+"""
+    current_scope()::Union{Nothing, Scope}
+
+Return the current dynamic scope.
+"""
+current_scope() = current_task().scope::Union{Nothing, Scope}
+
+function Base.show(io::IO, scope::Scope)
+    print(io, Scope, "(")
+    first = true
+    for (key, value) in scope.values
+        if first
+            first = false
+        else
+            print(io, ", ")
+        end
+        print(io, typeof(key), "@")
+        show(io, Base.objectid(key))
+        print(io, " => ")
+        show(IOContext(io, :typeinfo => eltype(key)), value)
+    end
+    print(io, ")")
+end
+
+struct NoValue end
+const novalue = NoValue()
+
+"""
+    get(val::ScopedValue{T})::Union{Nothing, Some{T}}
+
+If the scoped value isn't set and doesn't have a default value,
+return `nothing`. Otherwise returns `Some{T}` with the current
+value.
+"""
+function get(val::ScopedValue{T}) where {T}
+    # Inline current_scope to avoid doing the type assertion twice.
+    scope = current_task().scope
+    if scope === nothing
+        isassigned(val) && return Some(val.default)
+        return nothing
+    end
+    scope = scope::Scope
+    if isassigned(val)
+        return Some(Base.get(scope.values, val, val.default)::T)
+    else
+        v = Base.get(scope.values, val, novalue)
+        v === novalue || return Some(v::T)
+    end
+    return nothing
+end
+
+function Base.getindex(val::ScopedValue{T})::T where T
+    maybe = get(val)
+    maybe === nothing && throw(KeyError(val))
+    return something(maybe)::T
+end
+
+function Base.show(io::IO, val::ScopedValue)
+    print(io, ScopedValue)
+    print(io, '{', eltype(val), '}')
+    print(io, '(')
+    v = get(val)
+    if v === nothing
+        print(io, "undefined")
+    else
+        show(IOContext(io, :typeinfo => eltype(val)), something(v))
+    end
+    print(io, ')')
+end
+
+"""
+    with(f, (var::ScopedValue{T} => val::T)...)
+
+Execute `f` in a new scope with `var` set to `val`.
+"""
+function with(f, pair::Pair{<:ScopedValue}, rest::Pair{<:ScopedValue}...)
+    @nospecialize
+    ct = Base.current_task()
+    current_scope = ct.scope::Union{Nothing, Scope}
+    ct.scope = Scope(current_scope, pair, rest...)
+    try
+        return f()
+    finally
+        ct.scope = current_scope
+    end
+end
+
+with(@nospecialize(f)) = f()
+
+"""
+    @with vars... expr
+
+Macro version of `with(f, vars...)` but with `expr` instead of `f` function.
+This is similar to using [`with`](@ref) with a `do` block, but avoids creating
+a closure.
+"""
+macro with(exprs...)
+    if length(exprs) > 1
+        ex = last(exprs)
+        exprs = exprs[1:end-1]
+    elseif length(exprs) == 1
+        ex = only(exprs)
+        exprs = ()
+    else
+        error("@with expects at least one argument")
+    end
+    for expr in exprs
+        if expr.head !== :call || first(expr.args) !== :(=>)
+            error("@with expects arguments of the form `A => 2` got $expr")
+        end
+    end
+    exprs = map(esc, exprs)
+    quote
+        ct = $(Base.current_task)()
+        current_scope = ct.scope::$(Union{Nothing, Scope})
+        ct.scope = $(Scope)(current_scope, $(exprs...))
+        $(Expr(:tryfinally, esc(ex), :(ct.scope = current_scope)))
+    end
+end
+
+end # module ScopedValues

doc/make.jl

@@ -112,6 +112,7 @@ BaseDocs = [
     "base/arrays.md",
     "base/parallel.md",
     "base/multi-threading.md",
+    "base/scopedvalues.md",
     "base/constants.md",
     "base/file.md",
     "base/io-network.md",