add corner case tests

this hits 100%

src/utils.jl

@@ -31,15 +31,17 @@ function parse_formula(x::Maybe{String})
     return res
 end
 
-function parse_charge(x)
+function parse_charge(x)::Maybe{Int}
     if isa(x, Int)
-        return x
+        x
     elseif isa(x, Float64)
-        return Int(x)::Int
+        Int(x)
     elseif isa(x, String)
-        return parse(Int, x)
-    else
+        Int(parse(Float64, x))
+    elseif isnothing(x)
         nothing
+    else
+        throw(DomainError(x, "cannot parse charge"))
     end
 end
 

test/misc.jl

@@ -11,3 +11,15 @@
     # caps suffix is quite common
     @test all(suffix in A.filename_extensions(JSONFBCModel) for suffix in ["json", "JSON"])
 end
+
+@testset "Corner cases" begin
+    import JSONFBCModels: parse_charge
+
+    @test parse_charge(1) == 1
+    @test parse_charge(2.0) == 2
+    @test parse_charge("3") == 3
+    @test parse_charge("4.0") == 4
+    @test parse_charge(nothing) == nothing
+    @test_throws ArgumentError parse_charge("totally positive charge")
+    @test_throws DomainError parse_charge(["very charged"])
+end