WIP: rewrite gap_to_julia

TODO:
- add more test cases esp. for the various fixes
  e.g. converting the same (identical) objects to multiple
  output types should be working correctly now
- identify issues this patch resolves
- update docs
- ...

CHANGES.md

@@ -1,3 +1,19 @@
+## Version X.Y.Z-DEV (released YYYY-MM-DD)
+
+- Rewrite `gap_to_julia` to improve performance, fix bugs in many corner
+  cases, and make it easier to extend it correctly in other packages. The
+  target type of conversions now must be either ...
+  - `Any` (in which case we try to "guess" a suitable result type),
+  - or a supertype of the given object (in which case the input is returned unmodified),
+  - or else a concrete type (and this will then be the type of the result).
+
+  Thus e.g. `GAP.gap_to_julia(AbstractString, x)`  no longer
+  works if `x` is a GAP string object; use `GAP.gap_to_julia(String, x)`
+  instead.
+  To extend the GAP-to-Julia conversion code, you should provide a method
+  `gap_to_julia_intern(type::YourType, input::Any, recursive::Bool, dict::GapToJuliaDict)`
+  TODO: .. consult the documentation in location TODO
+
 # Changes in GAP.jl
 
 ## Version 0.9.8 (released 2023-09-11)

pkg/JuliaInterface/tst/convert.tst

@@ -255,7 +255,7 @@ gap> Julia.GAP.Obj( xx );
 
 ##  empty record
 gap> dict:= GAPToJulia( rec() );
-<Julia: Dict{Symbol,Any}()>
+<Julia: Dict{Symbol, Any}()>
 gap> JuliaToGAP( IsRecord, dict );
 rec(  )
 gap> Julia.GAP.Obj( dict );
@@ -278,7 +278,7 @@ rec( bool := true, list := [ 1, 2, 3 ], string := "abc" )
 ##  something where recursive conversion would run into a Julia error
 gap> dict:= GAPToJulia( rec( juliafunc:= Julia.Base.map,
 >                          ) );
-<Julia: Dict{Symbol,Any}(:juliafunc=>map)>
+<Julia: Dict{Symbol, Any}(:juliafunc => map)>
 gap> JuliaToGAP( IsRecord, dict );
 rec( juliafunc := <Julia: map> )
 gap> Julia.GAP.Obj( dict );

src/constructors.jl

@@ -254,7 +254,7 @@ function BitVector(obj::GapObj)
     !Wrappers.IsBlist(obj) && throw(ConversionError(obj, BitVector))
     # TODO: a much better conversion would be possible, at least if `obj` is
     # in IsBlistRep, then we could essentially memcpy data
-    len = Wrappers.Length(obj)
+    len = length(obj)
     result = BitVector(undef, len)
     for i = 1:len
         result[i] = obj[i]
@@ -465,7 +465,7 @@ julia> UnitRange{Int32}(val)
 
 function (::Type{T})(obj::GapObj) where {T<:UnitRange}
     !Wrappers.IsRange(obj) && throw(ConversionError(obj, T))
-    len = Wrappers.Length(obj)
+    len = length(obj)
     if len == 0
         # construct an empty UnitRange object
         result = 1:0
@@ -506,7 +506,7 @@ StepRange{Int8, Int8}
 
 function(::Type{T})(obj::GapObj) where {T<:StepRange}
     !Wrappers.IsRange(obj) && throw(ConversionError(obj, T))
-    len = Wrappers.Length(obj)
+    len = length(obj)
     if len == 0
         # construct an empty StepRange object
         start = 1