Decision Trees

Three trees, one page

The decision trees that appear in scattered form across the chapter intros are consolidated here. Use this page when you need to route quickly to the right tool.

Tree 1: Which optic do I need?

flowchart TD
    Q{"What are you doing<br/>to the focus?"}
    Q -->|"reading only"| R{"How many<br/>targets?"}
    Q -->|"reading and writing"| M{"How many<br/>targets?"}
    Q -->|"converting between<br/>equivalent types"| I(["Iso"])

    R -->|"exactly one"| G(["Getter"])
    R -->|"zero or more"| F(["Fold"])

    M -->|"exactly one"| L(["Lens"])
    M -->|"zero or one:<br/>the field may be absent"| A(["Affine"])
    M -->|"zero or one:<br/>the value may be another variant"| P(["Prism"])
    M -->|"zero or more"| T(["Traversal"])

    classDef decision fill:#e5c890,stroke:#df8e1d,color:#232634
    classDef tier fill:#a6d189,stroke:#40a02b,color:#232634
    class Q,R,M decision
    class I,G,F,L,A,P,T tier

Write-only access is the one case the tree does not reach: that is a Setter, and you arrive at it by knowing you never read.

Tree 2: Which API style?

flowchart TD
    S(["Start: Focus DSL<br/>CompanyFocus.headquarters().city()"]) --> Q{"Does the update<br/>need more?"}
    Q -->|"no: plain nested update"| S2(["stay on the Focus DSL"])
    Q -->|"it can fail, or accumulate errors"| FA(["Fluent API: OpticOps<br/>modifyEither, modifyAllValidated"])
    Q -->|"it must be inspected,<br/>audited or run several ways"| FM(["Free Monad DSL<br/>see Advanced Optics"])

    classDef decision fill:#e5c890,stroke:#df8e1d,color:#232634
    classDef tier fill:#a6d189,stroke:#40a02b,color:#232634
    classDef wire fill:#8caaee,stroke:#1e66f5,color:#232634
    class Q decision
    class S2,FA,FM tier
    class S wire

Tree 3: Which advanced feature?

flowchart TD
    Q{"What is the<br/>constraint?"}
    Q -->|"only some elements<br/>should be touched"| F(["Filtered optics"])
    Q -->|"the position matters<br/>as well as the value"| I(["Indexed optics"])
    Q -->|"the source or target<br/>is the wrong shape"| P(["Profunctor optics"])

    classDef decision fill:#e5c890,stroke:#df8e1d,color:#232634
    classDef tier fill:#a6d189,stroke:#40a02b,color:#232634
    class Q decision
    class F,I,P tier

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