Conversion of VarName to/from string

src/AbstractPPL.jl

@@ -8,6 +8,8 @@ export VarName,
     subsumes,
     subsumedby,
     varname,
+    vn_to_string,
+    vn_from_string,
     vsym,
     @varname,
     @vsym

src/varname.jl

@@ -302,7 +302,7 @@ subsumes(t::ComposedOptic, u::ComposedOptic) =
 # If `t` is still a composed lens, then there is no way it can subsume `u` since `u` is a
 # leaf of the "lens-tree".
 subsumes(t::ComposedOptic, u::PropertyLens) = false
-# Here we need to check if `u.outer` (i.e. the next lens to be applied from `u`) is
+# Here we need to check if `u.inner` (i.e. the next lens to be applied from `u`) is
 # subsumed by `t`, since this would mean that the rest of the composition is also subsumed
 # by `t`.
 subsumes(t::PropertyLens, u::ComposedOptic) = subsumes(t, u.inner)
@@ -450,6 +450,7 @@ struct ConcretizedSlice{T,R} <: AbstractVector{T}
 end
 
 ConcretizedSlice(s::Base.Slice{R}) where {R} = ConcretizedSlice{eltype(s.indices),R}(s.indices)
+ConcretizedSlice(s::Base.OneTo{R}) where {R} = ConcretizedSlice(Base.Slice(s))
 Base.show(io::IO, s::ConcretizedSlice) = print(io, ":")
 Base.show(io::IO, ::MIME"text/plain", s::ConcretizedSlice) =
     print(io, "ConcretizedSlice(", s.range, ")")
@@ -747,3 +748,110 @@ function vsym(expr::Expr)
         error("Malformed variable name `$(expr)`!")
     end
 end
+
+"""
+    index_to_str(i)
+
+Generates a string representation of the index `i`, or a tuple thereof.
+
+## Examples
+
+```jldoctest
+julia> index_to_str(2)
+"2"
+
+julia> index_to_str((1, 2:5))
+"(1, 2:5,)"
+
+julia> index_to_str(:)
+":"
+
+julia> index_to_str(ConcretizedSlice(Base.Slice(Base.OneTo(10))))
+"ConcretizedSlice(Base.OneTo(10))"
+```
+"""
+index_to_str(i::Integer) = string(i)
+index_to_str(r::UnitRange) = "$(first(r)):$(last(r))"
+index_to_str(::Colon) = ":"
+index_to_str(s::ConcretizedSlice{T,R}) where {T,R} = "ConcretizedSlice(" * repr(s.range) * ")"
+index_to_str(t::Tuple) = "(" * join(map(index_to_str, t), ", ") * ",)"
+
+"""
+    optic_to_nt(optic)
+
+Convert an optic to a named tuple representation.
+
+## Examples
+```jldoctest; setup=:(using Accessors)
+julia> optic_to_nt(identity)
+(type = "identity",)
+
+julia> optic_to_nt(@optic _.a)
+(type = "property", field = "a")
+
+julia> optic_to_nt(@optic _.a.b)
+(type = "composed", outer = (type = "property", field = "b"), inner = (type = "property", field = "a"))
+
+julia> optic_to_nt(@optic _[1])  # uses index_to_str()
+(type = "index", indices = "(1,)")
+```
+"""
+optic_to_nt(::typeof(identity)) = (type = "identity",)
+optic_to_nt(::PropertyLens{sym}) where {sym} = (type = "property", field = String(sym))
+optic_to_nt(i::IndexLens) = (type = "index", indices = index_to_str(i.indices))
+optic_to_nt(c::ComposedOptic) = (type = "composed", outer = optic_to_nt(c.outer), inner = optic_to_nt(c.inner))
+
+
+"""
+    nt_to_optic(nt)
+
+Convert a named tuple representation back to an optic.
+"""
+function nt_to_optic(nt)
+    if nt.type == "identity"
+        return identity
+    elseif nt.type == "index"
+        return IndexLens(eval(Meta.parse(nt.indices)))
+    elseif nt.type == "property"
+        return PropertyLens{Symbol(nt.field)}()
+    elseif nt.type == "composed"
+        return nt_to_optic(nt.outer) ∘ nt_to_optic(nt.inner)
+    end
+end
+
+"""
+    vn_to_string(vn::VarName)
+
+Convert a `VarName` as a string, via an intermediate named tuple. This differs
+from `string(vn)` in that concretised slices are faithfully represented (rather
+than being pretty-printed as colons).
+
+```jldoctest
+julia> vn_to_string(@varname(x))
+"(sym = \"x\", optic = (type = \"identity\",))"
+
+julia> vn_to_string(@varname(x.a))
+"(sym = \"x\", optic = (type = \"property\", field = \"a\"))"
+
+julia> y = ones(2); vn_to_string(@varname(y[:]))
+"(sym = \"y\", optic = (type = \"index\", indices = \"(:,)\"))"
+
+julia> y = ones(2); vn_to_string(@varname(y[:], true))
+"(sym = \"y\", optic = (type = \"index\", indices = \"(ConcretizedSlice(Base.OneTo(2)),)\"))"
+```
+"""
+vn_to_string(vn::VarName) = repr((sym = String(getsym(vn)), optic = optic_to_nt(getoptic(vn))))
+
+"""
+    vn_from_string(str)
+
+Convert a string representation of a `VarName` back to a `VarName`. The string
+should have been generated by `vn_to_string`.
+
+NOTE: This function should only be used with trusted input, as it uses `eval`
+and `Meta.parse` to parse the string.
+"""
+function vn_from_string(str)
+    new_fields = eval(Meta.parse(str))
+    return VarName{Symbol(new_fields.sym)}(nt_to_optic(new_fields.optic))
+end

test/varname.jl

@@ -137,4 +137,32 @@ end
         @inferred get(c, @varname(b.a[1]))
         @inferred Accessors.set(c, @varname(b.a[1]), 10)
     end
+
+    @testset "roundtrip conversion to/from string" begin
+        y = ones(10)
+        vns = [
+            @varname(x),
+            @varname(ä),
+            @varname(x.a),
+            @varname(x.a.b),
+            @varname(var"x.a"),
+            @varname(x[1]),
+            @varname(var"x[1]"),
+            @varname(x[1:10]),
+            @varname(x[1, 2]),
+            @varname(x[1, 2:5]),
+            @varname(x[:]),
+            @varname(x.a[1]),
+            @varname(x.a[1:10]),
+            @varname(x[1].a),
+            @varname(y[:]),
+            @varname(y[begin:end]),
+            @varname(y[end]),
+            @varname(y[:], false),
+            @varname(y[:], true),
+        ]
+        for vn in vns
+            @test vn_from_string(vn_to_string(vn)) == vn
+        end
+    end
 end