···

       
307
       
 
       


     

       
308
       
 
       
## Sum types {#sec-option-types-sums}

     

       
309
       
 
       


     

       
310
       
-
       
A sum type can be thought of, conceptually, as a *`types.enum`* where each valid item is paired with a type, through some value syntax.

     

       
311
       
 
       
Nix does not have a built-in syntax for this pairing of a label and a type or value, so sum types may be represented in multiple ways.

     

       
312
       
 
       


     

       
313
       
-
       
If the built-in Nix value types provide enough distinction, you simplify your syntax with a [union type](#sec-option-types-unions) instead.

     

       
314
       
 
       


     

       
315
       
-
       
`types.attrTag` *`{ attr1 = t1; attr2 = t2; ... }`*

     

       
316
       
 
       


     

       
317
       
 
       
:   An attribute set containing one attribute, whose name must be picked from

     

       
318
       
-
       
    the attribute set (`attr1`, etc) and whose value must be of the accompanying

     

       
319
       
-
       
    type.

     

       
320
       
 
       


     

       
321
       
 
       
    This type appears in the documentation as _attribute-tagged union_.

     

       
322
       
 
       


     

···

       
307
       
 
       


     

       
308
       
 
       
## Sum types {#sec-option-types-sums}

     

       
309
       
 
       


     

       
310
       
+
       
A sum type can be thought of, conceptually, as a *`types.enum`* where each valid item is paired with at least a type, through some value syntax.

     

       
311
       
 
       
Nix does not have a built-in syntax for this pairing of a label and a type or value, so sum types may be represented in multiple ways.

     

       
312
       
 
       


     

       
313
       
+
       
If the you're interested in can be distinguished without a label, you may simplify your value syntax with a [union type](#sec-option-types-unions) instead.

     

       
314
       
 
       


     

       
315
       
+
       
`types.attrTag` *`{ attr1 = option1; attr2 = option2; ... }`*

     

       
316
       
 
       


     

       
317
       
 
       
:   An attribute set containing one attribute, whose name must be picked from

     

       
318
       
+
       
    the attribute set (`attr1`, etc) and whose value consists of definitions that are valid for the corresponding option (`option1`, etc).

     

       
0
       
 
       

     

       
319
       
 
       


     

       
320
       
 
       
    This type appears in the documentation as _attribute-tagged union_.

     

       
321