Fix use of norm and remove vecnorm

The behavior of `norm(x, p)` for matrices changed in Julia PR 27401 to
be the same as `norm(vec(x), p)` and `vecnorm` was removed. The previous
behavior of `norm` for matrices is now captured by `opnorm`. This
updates our extensions to these functions to match, and emits
deprecation warnings as appropriate.

As a somewhat related change, this also deprecates the positional
dimension argument to `sum` in favor of keyword arguments, in keeping
with Base.

src/atoms/affine/sum.jl

@@ -57,9 +57,13 @@ function conic_form!(x::SumAtom, unique_conic_forms::UniqueConicForms=UniqueConi
   return get_conic_form(unique_conic_forms, x)
 end
 
-sum(x::AbstractExpr) = SumAtom(x)
+# Dispatch to an internal helper function that handles the dimension argument in
+# the same manner as Base, with dims=: denoting a regular sum
+sum(x::AbstractExpr; dims=:) = _sum(x, dims)
 
-function sum(x::AbstractExpr, dimension::Int)
+_sum(x::AbstractExpr, ::Colon) = SumAtom(x)
+
+function _sum(x::AbstractExpr, dimension::Integer)
   if dimension == 1
     return Constant(ones(1, x.size[1]), Positive()) * x
   elseif dimension == 2
@@ -68,3 +72,5 @@ function sum(x::AbstractExpr, dimension::Int)
     error("Sum not implemented for dimension $dimension")
   end
 end
+
+Base.@deprecate sum(x::AbstractExpr, dim::Int) sum(x, dims=dim)

src/atoms/sdp_cone/operatornorm.jl

@@ -5,6 +5,7 @@
 # All expressions and atoms are subtypes of AbstractExpr.
 # Please read expressions.jl first.
 #############################################################################
+import LinearAlgebra: opnorm
 export operatornorm, sigmamax
 
 ### Operator norm
@@ -42,6 +43,21 @@ end
 operatornorm(x::AbstractExpr) = OperatorNormAtom(x)
 sigmamax(x::AbstractExpr) = OperatorNormAtom(x)
 
+function opnorm(x::AbstractExpr, p::Real=2)
+  if length(size(x)) <= 1 || minimum(size(x)) == 1
+    throw(ArgumentError("argument to `opnorm` must be a matrix"))
+  end
+  if p == 1
+    return maximum(sum(abs(x), dims=1))
+  elseif p == 2
+    return operatornorm(x)
+  elseif p == Inf
+    return maximum(sum(abs(x), dims=2))
+  else
+    throw(ArgumentError("matrix p-norms only defined for p = 1, 2, and Inf"))
+  end
+end
+
 # Create the equivalent conic problem:
 #   minimize t
 #   subject to

src/atoms/second_order_cone/norm.jl

@@ -1,5 +1,5 @@
 import LinearAlgebra.norm
-export norm_inf, norm, norm_1, vecnorm
+export norm_inf, norm, norm_1
 
 # deprecate these soon
 norm_inf(x::AbstractExpr) = maximum(abs(x))
@@ -25,19 +25,16 @@ function norm(x::AbstractExpr, p::Real=2)
       error("vector p-norms not defined for p < 1")
     end
   else
-    # x is a matrix
-    if p == 1
-      return maximum(sum(abs(x), 1))
-    elseif p == 2
-      return operatornorm(x)
-    elseif p == Inf
-      return maximum(sum(abs(x), 2))
-    else
-      error("matrix p-norms only defined for p = 1, 2, and Inf")
-    end
+    # TODO: After the deprecation period, allow this again but make it consistent with
+    # LinearAlgebra, i.e. make norm(x, p) for x a matrix the same as norm(vec(x), p).
+    Base.depwarn("`norm(x, p)` for matrices will in the future be equivalent to " *
+                 "`norm(vec(x), p)`. Use `opnorm(x, p)` for the Julia 0.6 behavior of " *
+                 "computing the operator norm for matrices.", :norm)
+    return opnorm(x, p)
   end
 end
 
-function vecnorm(x::AbstractExpr, p::Real=2)
-  return norm(vec(x), p)
+if isdefined(LinearAlgebra, :vecnorm) # deprecated but defined
+  import LinearAlgebra: vecnorm
 end
+Base.@deprecate vecnorm(x::AbstractExpr, p::Real=2) norm(vec(x), p)

src/atoms/second_order_cone/norm2.jl

@@ -5,7 +5,7 @@
 # Please read expressions.jl first.
 #############################################################################
 import LinearAlgebra.norm2
-export EucNormAtom, norm2, vecnorm
+export EucNormAtom, norm2
 export sign, monotonicity, curvature, conic_form!
 
 

src/expressions.jl

@@ -4,7 +4,7 @@
 # Each type which subtypes AbstractExpr (Variable and Constant being exceptions)
 # must have:
 #
-## head::Symbol                  -- a symbol such as :vecnorm, :+ etc
+## head::Symbol                  -- a symbol such as :norm, :+ etc
 ## children::(AbstractExpr,)     -- The expressions on which the current expression
 ##                               -- is operated
 ## id_hash::UInt64               -- identifier hash, can be a hash of children

test/test_socp.jl

@@ -49,11 +49,11 @@ TOL = 1e-3
   @testset "frobenius norm atom" begin
     m = Variable(4, 5)
     c = [m[3, 3] == 4, m >= 1]
-    p = minimize(vecnorm(m, 2), c)
+    p = minimize(norm(vec(m), 2), c)
     @test vexity(p) == ConvexVexity()
     solve!(p)
     @test p.optval ≈ sqrt(35) atol=TOL
-    @test evaluate(vecnorm(m, 2)) ≈ sqrt(35) atol=TOL
+    @test evaluate(norm(vec(m), 2)) ≈ sqrt(35) atol=TOL
   end
 
   @testset "quad over lin atom" begin
@@ -226,18 +226,18 @@ TOL = 1e-3
     @test norm(g, 2) ^ 2 ≈ 0 atol=TOL
   end
 
-  @testset "norm consistent with Base" begin
+  @testset "matrix norm consistent with Base" begin
     A = randn(4, 4)
     x = Variable(4, 4)
     x.value = A
-    @test evaluate(norm(x)) ≈ opnorm(A) atol=TOL
-    @test evaluate(norm(x, 1)) ≈ opnorm(A, 1) atol=TOL
-    @test evaluate(norm(x, 2)) ≈ opnorm(A, 2) atol=TOL
-    @test evaluate(norm(x, Inf)) ≈ opnorm(A, Inf) atol=TOL
-    @test evaluate(vecnorm(x, 1)) ≈ norm(vec(A), 1) atol=TOL
-    @test evaluate(vecnorm(x, 2)) ≈ norm(vec(A), 2) atol=TOL
-    @test evaluate(vecnorm(x, 7)) ≈ norm(vec(A), 7) atol=TOL
-    @test evaluate(vecnorm(x, Inf)) ≈ norm(vec(A), Inf) atol=TOL
+    @test evaluate(opnorm(x)) ≈ opnorm(A) atol=TOL
+    @test evaluate(opnorm(x, 1)) ≈ opnorm(A, 1) atol=TOL
+    @test evaluate(opnorm(x, 2)) ≈ opnorm(A, 2) atol=TOL
+    @test evaluate(opnorm(x, Inf)) ≈ opnorm(A, Inf) atol=TOL
+    @test evaluate(norm(vec(x), 1)) ≈ norm(vec(A), 1) atol=TOL
+    @test evaluate(norm(vec(x), 2)) ≈ norm(vec(A), 2) atol=TOL
+    @test evaluate(norm(vec(x), 7)) ≈ norm(vec(A), 7) atol=TOL
+    @test evaluate(norm(vec(x), Inf)) ≈ norm(vec(A), Inf) atol=TOL
   end