parse query on Select object creation (#61)

* parse query on Select object creation

* fix Select test: the macro sel throws a `LoadError` before the `ArgumentError`.

* remove unnecessary object creation in test

src/select.jl

@@ -436,11 +436,16 @@ allselector(el) = true
 
 # Acts as a function when used within typical julia filtering functions 
 #   by converting a string selection into a query call
-struct Select <: Function
-    sel::String
+struct Select{Q} <: Function
+    query_string::String
+    query::Q
 end
-
-(s::Select)(at) = apply_query(parse_query(s.sel), at)
+function Select(query_string::AbstractString) 
+    query = parse_query(query_string)
+    return Select(query_string, query)
+end
+(s::Select)(at) = apply_query(s.query, at)
+Base.show(io::IO, ::MIME"text/plain", s::Select) = print(io, """Select("$(s.query_string)")""")
 
 "String selection syntax."
 macro sel_str(str)

test/runtests.jl

@@ -937,9 +937,14 @@ end
     @test length(collectmodels(struc, sel"model 1")) == 1
     @test length(collectmodels(struc, sel"model 2")) == 0
 
-    @test_throws ArgumentError collectatoms(struc, sel"abc") # Invalid selection syntax
-    @test_throws ArgumentError collectatoms(struc, sel"index = A") # Invalid value type
-    @test_throws ArgumentError collectatoms(struc, sel"resnum C")
+    @test_throws ArgumentError collectatoms(struc, BioStructures.Select("abc")) # Invalid selection syntax
+    @test_throws ArgumentError collectatoms(struc, BioStructures.Select("index = A")) # Invalid value type
+    @test_throws ArgumentError collectatoms(struc, BioStructures.Select("resnum C"))
+
+    # test show method for @sel_str
+    buff = IOBuffer()
+    show(buff, MIME"text/plain"(), sel"name CA and resnum 1")
+    @test String(take!(buff)) == """Select("name CA and resnum 1")"""
 end
 
 @testset "PDB reading" begin