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    A simple library for complex logic.

    What is it

    A series of tiny modules that make encoding logic as data a breeze ⛱️.


    • fold A module of utility functions that interop with other static-sum-type modules.

    • predicated Generates spec compliant unions while also allowing you to specify a predicate that the value must satisfy in order for an error to not be handled.

    • yslashn Generates spec compliant unions that mimic well known types like maybe and either with a terse Y/N naming convention.

    • taggy Generates spec compliant unions that also verify the existence of properties for each case.

    Each module adheres to the static-sum-type specification. That specification is defined in static-sum-type/fold.

    The design of the spec allows for a variety of other statically analyzable syntaxes which can be viewed at static-sum-type/fold. But its also possible to generate types dynamically far more succintly.

    Project Goals and Motivations

    • 0 Dependencies
    • Tiny for frontend usage
    • Data oriented precision
    • Serializable
    • A simple and convenient abstraction for library interop and direct usage.

    How does static-sum-type differ from other libraries in the ecosystem.

    static-sum-type removes the following features because we believe they lead to brittle codebases.

    • placeholder cases
    • auto spreading of values in cata/fold
    • auto curried constructors
    • prototypes

    static-sum-type is technically 0KB, it's an idea. You can use static-sum-type in your codebase without ever running npm install.


    What's a "Type"

    A type is a struct with a name property and as many keys as there are cases.

    Each case on the type must have a name property that matches it's key.

    { name :: TitlecaseString
    , [a] :: any
    , [b] :: any

    Here is an example of a valid Maybe type

    { name: 'Maybe' 
    , Just: null
    , Nothing: null

    Keep in mind lowercase properties other than name are ignored. If you want your type to have static functions or other data you can safely do so as long as the property is a LowercaseString You can safely add static methods or properties to your type structure without interfing with static-sum-type provided they do not start with a capital letter.

    💡 Any keys in a call to getOwnPropertyNames(Type) where key[0] == key[0].toUpperCase() will be treated as a case.

    Because .name is auto generated for functions and classes, the following forms are spec compliant.

    const Maybe = {
        name: 'Maybe'
        Just(){} // == 'Just'
        Nothing(){} // == 'Nothing'
    class Maybe {
        static Just(){} // == 'Just'
        static Nothing(){} // == 'Nothing'

    What is a "Case"

    { type :: TitlecaseString
    , case :: TitlecaseString
    , value? :: a
    • A Case is a struct with a type, case and an optional value property.
    • The type and case property must be an TitlecaseString.
    • The case property must correspond to a matching key on a type object.
    • The matching type object must include a property that matches the cases case property.

    A case can have a value property. But it is optional. The value property can be of any type.

    💡 The above specification is compatible with Javascript classes because class has an automatically generated property name.

    Show me some examples of some valid cases.

    { case: 'Case1'
    , type: //references above
    { case: 'Case2'
    , type: 'Type1' // has same name as above which is also OK
    , value: 'this is a value'


    npm i static-sum-type

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