···

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

     

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

     

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

     

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

     

       
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+
       
    type.

     

       
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## Composed types {#sec-option-types-composed}

     
···

       
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:   Ensures that type *`t`* cannot be merged. Prints the message *`m`*, after

     

       
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    the line `The option <option path> is defined multiple times.` and before

     

       
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    a list of definition locations.

     

       
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-
       


     

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

     

       
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-
       


     

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

     

       
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371
       
-
       
    the attribute set (`attr1`, etc) and whose value must be of the accompanying

     

       
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-
       
    type.

     

       
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-
       


     

       
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-
       
    This is one possible representation of what may be called a _tagged union_ or _sum type_.

     

       
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-
       
    `attrTag` can be thought of as an extension of *`enum`* where the permissible items

     

       
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-
       
    are attribute names, and each item is paired with a value of a specific type.

     

       
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`types.coercedTo` *`from f to`*

     

       
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