Turn hash for GapObj into error

CHANGES.md

@@ -1,5 +1,13 @@
 # Changes in GAP.jl
 
+## Version 0.10.0-DEV (released YYYY-MM-DD)
+
+- Changed `hash(::GapObj)` to throw an error (no general non-trivial hashing is
+  possible for general GAP objects)
+- Remove support for conversion to `AbstractString` (it was not meaningful anyway,
+  and hopefully nobody used it; but if you did, just convert to `String` instead
+  for an identical outcome)
+
 ## Version 0.9.8 (released 2023-09-11)
 
 - Allow GAP.Obj(x,true) for recursive conversion (#910. #925)

Project.toml

@@ -1,7 +1,7 @@
 name = "GAP"
 uuid = "c863536a-3901-11e9-33e7-d5cd0df7b904"
 authors = ["Thomas Breuer <[email protected]>", "Sebastian Gutsche <[email protected]>", "Max Horn <[email protected]>"]
-version = "0.9.8"
+version = "0.10.0-DEV"
 
 [deps]
 Artifacts = "56f22d72-fd6d-98f1-02f0-08ddc0907c33"

src/adapter.jl

@@ -361,11 +361,12 @@ for (left, right) in typecombinations
 end
 
 # Since we define the equality of GAP objects (see above),
-# we must provide a safe `hash` method, otherwise `==` results may be wrong.
+# we must provide a safe `hash` method, otherwise using GAP objects
+# in dictionaries or `Set`s can lead to inconsistent results.
 # For example, `==` for `Set`s of GAP objects may erroneously return
 # `false` if the default `hash` is used.
-Base.hash(::GapObj, h::UInt) = h
-Base.hash(::FFE, h::UInt) = h
+Base.hash(::GapObj, h::UInt) = error("hash for GAP objects is not implemented")
+Base.hash(::FFE, h::UInt) = error("hash for GAP objects is not implemented")
 
 ### RNGs
 

src/constructors.jl

@@ -389,23 +389,6 @@ Set{Any} with 2 elements:
   Any[1]
   Any[2]
 ```
-
-In the following examples,
-the order in which the Julia output is shown may vary.
-
-# Examples
-```jldoctest
-julia> s = Set{Any}(GAP.Obj([[1], [2], [1]]; recursive=true); recursive=false);
-
-julia> s == Set{Any}([GAP.Obj([1]), GAP.Obj([2])])
-true
-
-julia> s = Set{Any}(GAP.Globals.SymmetricGroup(2); recursive=false);
-
-julia> s == Set{Any}([GAP.evalstr("()"), GAP.evalstr("(1,2)")])
-true
-
-```
 """
 Base.Set{T}(obj::GapObj; recursive::Bool = true) where {T} =
     gap_to_julia(Set{T}, obj; recursive)

test/basics.jl

@@ -28,21 +28,13 @@
     y = GAP.evalstr("[]")
     @test !(x === y)
     @test (x == y)
-    @test hash(x) == 0
+    @test_throws ErrorException hash(x)
 
     x = GAP.evalstr("Z(2)")
     y = GAP.evalstr("Z(4)^3")
     @test !(x === y)
     @test (x == y)
-    @test hash(x) == 0
-
-    # The following would sometimes fail if no dedicated `hash`
-    # method would be available for GAP objects.
-    for i in 1:100
-        x = Set{Any}([GAP.evalstr("()"), GAP.evalstr("(1,2)")])
-        y = Set{Any}([GAP.evalstr("()"), GAP.evalstr("(1,2)")])
-        @test (x == y)
-    end
+    @test_throws ErrorException hash(x)
 end
 
 @testset "globals" begin

test/constructors.jl

@@ -149,7 +149,7 @@
     @test (@inferred Set{Vector{Int}}(GAP.evalstr("[[1,2],[2,3,4]]"))) == Set([[1, 2], [2, 3, 4]])
     @test (@inferred Set{String}(GAP.evalstr("[\"b\", \"a\", \"b\"]"))) == Set(["b", "a", "b"])
     x = GAP.evalstr("SymmetricGroup(3)")
-    @test (@inferred Set{GAP.GapObj}(x)) == Set{GAP.GapObj}(GAP.Globals.AsSet(x))
+    #@test (@inferred Set{GAP.GapObj}(x)) == Set{GAP.GapObj}(GAP.Globals.AsSet(x))
   end
 
   @testset "Tuples" begin

test/conversion.jl

@@ -173,12 +173,12 @@
     x = GAP.evalstr("[ [ 1 ], [ 2 ], [ 1 ] ]")
     y = [GAP.evalstr("[ 1 ]"), GAP.evalstr("[ 2 ]")]
     @test (@inferred GAP.gap_to_julia(Set{Vector{Int}}, x)) == Set([[1], [2], [1]])
-    @test @inferred GAP.gap_to_julia(Set{GAP.GapObj}, x, recursive = false) == Set(y)
-    @test @inferred GAP.gap_to_julia(Set{Any}, x, recursive = false) == Set(y)
+    #@test @inferred GAP.gap_to_julia(Set{GAP.GapObj}, x, recursive = false) == Set(y)
+    #@test @inferred GAP.gap_to_julia(Set{Any}, x, recursive = false) == Set(y)
     @test (@inferred GAP.gap_to_julia(Set{Any}, x)) == Set([[1], [2], [1]])
     x = GAP.evalstr("[ Z(2), Z(3) ]")  # a non-collection
     y = [GAP.evalstr("Z(2)"), GAP.evalstr("Z(3)")]
-    @test GAP.gap_to_julia(Set{GAP.FFE}, x) == Set(y)
+    #@test GAP.gap_to_julia(Set{GAP.FFE}, x) == Set(y)
   end
 
   @testset "Tuples" begin