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
- update CHANGES,md
- ...

CHANGES.md

@@ -1,11 +1,19 @@
 # Changes in GAP.jl
 
+<<<<<<< HEAD
 ## Version 0.8.0-DEV (released YYYY-MM-DD)
 
 - Upgrade to newer GAP snapshot
-- Give helpful error if GAP_jll is not available
+- Give helpful error if `GAP_jll` is not available
 - Remove `Base.big(obj::GapObj)`: nothing was using it and it does not really
   fit conceptually into this package.
+- Rewrite `gap_to_julia` to improve performance, fix bugs in many corner
+  cases, and making it easier to extend it correctly in other packages. The
+  target type of conversions now must be either `Any`, or a supertype of the
+  given object (in which case the input is returned unmodified), or else a
+  concrete type. 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.
 
 ## Version 0.7.7 (released 2022-02-14)
 
@@ -40,7 +48,7 @@
 
 ## Version 0.7.2 (released 2021-11-17)
 
-- Use a `GAP_pkg_juliainterface_jll` to installed a compiled version of the
+- Use `GAP_pkg_juliainterface_jll` to install a compiled version of the
   bundled C code, thus for basic use of GAP.jl no C/C++ compiler is needed
   anymore; this also avoids compatibility issues when switching back and forth
   between Julia 1.6 and 1.7

pkg/JuliaInterface/tst/convert.tst

@@ -133,7 +133,7 @@ gap> ForAll([0..64], n -> JuliaToGAP( IsInt, -big2^n) = -2^n);
 true
 
 #
-gap> string := GAPToJulia( Julia.Base.AbstractString, "bla" );
+gap> string := GAPToJulia( Julia.Base.String, "bla" );
 <Julia: "bla">
 gap> JuliaToGAP( IsString, string );
 "bla"
@@ -175,7 +175,7 @@ Error, <obj> must be a Julia range
 ##  empty list vs. empty string
 gap> emptylist:= GAPToJulia( JuliaEvalString( "Vector{Any}"), [] );
 <Julia: Any[]>
-gap> emptystring:= GAPToJulia( Julia.Base.AbstractString, "" );
+gap> emptystring:= GAPToJulia( Julia.Base.String, "" );
 <Julia: "">
 gap> JuliaToGAP( IsList, emptylist );
 [  ]
@@ -201,7 +201,7 @@ gap> JuliaToGAP( IsInt, xx );
 
 ##  empty record
 gap> dict:= GAPToJulia( rec() );
-<Julia: Dict{Symbol,Any}()>
+<Julia: Dict{Symbol, Any}()>
 gap> JuliaToGAP( IsRecord, dict );
 rec(  )
 
@@ -218,7 +218,7 @@ 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> )
 

src/constructors.jl

@@ -207,7 +207,6 @@ function String(obj::GapObj)
     Wrappers.IsString(obj) && return CSTR_STRING(Wrappers.CopyToStringRep(obj))
     throw(ConversionError(obj, String))
 end
-(::Type{T})(obj::GapObj) where {T<:AbstractString} = convert(T, String(obj))
 
 """
     Symbol(obj::GapObj)
@@ -250,7 +249,7 @@ 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]
@@ -457,7 +456,7 @@ 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
@@ -498,7 +497,7 @@ 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