PROOF OF CONCEPT: a function to write extension functions with warning

src/io.jl

@@ -7,6 +7,55 @@ WriteVTK.VTKCellType(::Type{<:Quadrilateral2D}) = VTKCellTypes.VTK_QUAD
 WriteVTK.VTKCellType(::Type{<:Tetrahedron3D}) = VTKCellTypes.VTK_TETRA
 WriteVTK.VTKCellType(::Type{<:Hexahedron3D}) = VTKCellTypes.VTK_HEXAHEDRON
 
+struct ExtensionLoadError <: Exception
+    fname::Symbol
+    weakdeps::Vector{Symbol}
+end
+
+Base.showerror(io::IO, e::ExtensionLoadError) =
+    print(
+    io,
+    "ExtensionLoadError: "
+        * "The function " * String(e.fname) * " is part of an extension.\n"
+        * "Suggestion: Install and load the following modules:"
+        * mapreduce(x -> "\n" * x, *, String.(e.weakdeps))
+)
+
+struct ExtensionMethodError <: Exception
+    fn::Function
+    args::Tuple
+end
+
+
+function Base.showerror(io::IO, e::ExtensionMethodError)
+    fname = String(nameof(e.fn))
+    argtypes = typeof.(e.args)
+    println(io,
+        "ExtensionMethodError: no method matching " * fname
+            * "(" * join(map(x-> "::"*String(nameof(x)),argtypes),',')
+            * ")")
+    println(io,"The function " * fname * " exists in a loaded extension," 
+            *" but no method is defined for this combination of argument types.\n"
+            * "\nCandidates are:\n"
+    )
+    println.(io, filter(x -> x != which(e.fn, (argtypes...,)), methods(e.fn)))
+    return nothing
+end
+
+function prepare_extension_function(fname::Symbol, weakdeps::Vector{Symbol}, args::Vector{Symbol})
+    expr = quote
+        function $fname($(args...); kwargs...)
+            if mapreduce(x -> isdefined(Main, x) && isa(getfield(Main,x),Module), &, $weakdeps)
+                throw(ExtensionMethodError($fname, ($(args...),)))
+            else
+                throw(ExtensionLoadError(nameof($fname), $weakdeps))
+            end
+            return nothing
+        end
+    end
+    return eval(expr)
+end
+
 """
 $(TYPEDSIGNATURES)
 
@@ -162,19 +211,11 @@ function simplexgrid(file::String; format = "", kwargs...)
 end
 
 function simplexgrid(file::String, ::Type{Val{:msh}}; kwargs...)
-    return try
-        simplexgrid_from_gmsh(file)
-    catch e
-        throw(ErrorException("Missing Gmsh extension. Add Gmsh.jl to your environment and import it to read msh files."))
-    end
+    return simplexgrid_from_gmsh(file)
 end
 
 function simplexgrid(file::String, ::Type{Val{:geo}}; kwargs...)
-    return try
-        simplexgrid_from_gmsh(file)
-    catch e
-        throw(ErrorException("Missing Gmsh extension. Add Gmsh.jl to your environment and import it to read geo files."))
-    end
+    return simplexgrid_from_gmsh(file)
 end
 
 function simplexgrid(file::String, ::Type{Val{:sg}}; kwargs...)
@@ -272,7 +313,11 @@ function simplexgrid(file::String, ::Type{Val{:sg}}; kwargs...)
     return g
 end
 
-function simplexgrid_from_gmsh end
+prepare_extension_function(:simplexgrid_from_gmsh, [:Gmsh], [:filename])
+# function simplexgrid_from_gmsh(filename; incomplete = false, Tc = Float32, Ti = Int32)
+#     throw(ErrorException("Missing Gmsh extension. Add Gmsh.jl to your environment and import it to read msh or geo files."))
+# end
+
 
 function simplexgrid_to_gmsh end