Implement SA[...] syntax for SArray literals

Upside: This gives extremely attractive syntax for short static array
literals.

Downside: This is a syntax pun: the result of the `SA[...]` depends on
whether the name `SA` is bound to `StaticArrays.SA` or is local variable
of some other type.

However, this is no worse than Int[1,2,3], which is also a pun on the
array indexing syntax. Cases where this syntax could refer to something
other than StaticArrays.SA are detectable by the compiler and in
principle also accessible to a linter.

Author: c42f

src/StaticArrays.jl

@@ -36,6 +36,7 @@ export SHermitianCompact
 
 export Size, Length
 
+export SA
 export @SVector, @SMatrix, @SArray
 export @MVector, @MMatrix, @MArray
 
@@ -113,6 +114,7 @@ include("SizedArray.jl")
 include("SDiagonal.jl")
 include("SHermitianCompact.jl")
 
+include("initializers.jl")
 include("convert.jl")
 
 include("abstractarray.jl")

src/initializers.jl

@@ -0,0 +1,38 @@
+"""
+    SA[ array initializer ]
+
+A type for initializing static array literals using array construction syntax.
+Returns an `SVector` or `SMatrix`.
+
+# Examples:
+
+* `SA[x, y]` creates a length-2 SVector
+* `SA[a b; c d]` creates a 2×2 SMatrix
+* `SA[a b]` creates a 1×2 SMatrix
+"""
+struct SA ; end
+
+Base.getindex(::Type{SA}, xs...) = SVector(xs)
+Base.typed_vcat(::Type{SA}, xs::Number...) = SVector(xs)
+Base.typed_hcat(::Type{SA}, xs::Number...) = SMatrix{1,length(xs)}(xs)
+
+Base.@pure function _SA_hvcat_transposed_type(rows)
+    M = rows[1]
+    if any(r->r != M, rows)
+        # @pure may not throw... probably. See
+        # https://discourse.julialang.org/t/can-pure-functions-throw-an-error/18459
+        return SMatrix{-1,-1}
+    end
+    SMatrix{M,length(rows)}
+end
+
+@inline function Base.typed_hvcat(::Type{SA}, rows::Dims, xs::Number...)
+    mtype = _SA_hvcat_transposed_type(rows)
+    if mtype === SMatrix{-1,-1}
+        throw(ArgumentError("SA[...] matrix shape inconsistent"))
+    end
+    # hvcat lowering is row major ordering, so must transpose
+    transpose(mtype(xs))
+end
+
+

test/initializers.jl

@@ -0,0 +1,18 @@
+SA_test_ref(x) = SA[1,x,x]
+@test @inferred(SA_test_ref(2))   === SVector{3,Int}((1,2,2))
+@test @inferred(SA_test_ref(2.0)) === SVector{3,Float64}((1,2,2))
+
+SA_test_vcat(x) = SA[1;x;x]
+@test @inferred(SA_test_vcat(2))   === SVector{3,Int}((1,2,2))
+@test @inferred(SA_test_vcat(2.0)) === SVector{3,Float64}((1,2,2))
+
+SA_test_hcat(x) = SA[1 x x]
+@test @inferred(SA_test_hcat(2))   === SMatrix{1,3,Int}((1,2,2))
+@test @inferred(SA_test_hcat(2.0)) === SMatrix{1,3,Float64}((1,2,2))
+
+SA_test_hvcat(x) = SA[1 x x;
+                      x 2 x]
+@test @inferred(SA_test_hvcat(3))   === SMatrix{2,3,Int}((1,3,3,2,3,3))
+@test @inferred(SA_test_hvcat(3.0)) === SMatrix{2,3,Float64}((1,3,3,2,3,3))
+@test @inferred(SA_test_hvcat(1.0im)) === SMatrix{2,3,ComplexF64}((1,1im,1im,2,1im,1im))
+

test/runtests.jl

@@ -31,6 +31,7 @@ include("convert.jl")
 include("core.jl")
 include("abstractarray.jl")
 include("indexing.jl")
+include("initializers.jl")
 Random.seed!(42); include("mapreduce.jl")
 Random.seed!(42); include("arraymath.jl")
 include("broadcast.jl")