# A toy that listens From A Little Lisp for a Small World, the Plazmi Lisp book (https://plazmi.gg/book). Checked against Plazmi Lisp version 2, 12 November 2026. The whole book as one file: https://plazmi.gg/book.md A verb waits to be played. A toy that listens hears what's said near it, and can answer. This chapter makes a parrot, a riddle stone and a mirror, and shows how to be good company: a toy that answers everything soon wears thin. ## :on-hear A toy with `:on-hear` hears every line said at your home, by you, a neighbour or a visitor. It is a function of three values: the toy, who spoke, and what they said: ```clojure (defobject parrot {:name "Parrot" :on-hear (fn [self who text] (when (includes? (lower text) "cracker") (say self "Cracker! Cracker!")))}) ``` ```text The parrot is ready. ``` ```text > say would you like a cracker [Juniper's parrot] Cracker! Cracker! > say lovely weather ``` The parrot heard both lines and answered one. `lower` makes the text small letters first, so "Cracker" and "CRACKER" are heard too. ## Spoken to, or overheard Words can be said to a toy (`say hello to the parrot`, or `say to the parrot: hello`) or near it, to nobody in particular. A toy can tell: `(:spoken-to self)` is `true` when the words were said to it. And when they were said to someone or something else, `(:to self)` says whom. A toy that answers only when it's spoken to stays out of other people's conversations: ```clojure (defobject riddle-stone {:name "Riddle stone" :state {:answer "echo"} :verbs {:tap (fn [self who] (say self "I answer every call and never speak first. What am I?"))} :on-hear (fn [self who text] (when (:spoken-to self) (if (includes? (lower text) (:answer self)) (say self "An echo. That's the one.") (say self "Not that one. Think of a valley."))))}) ``` ```text The riddle stone is ready. ``` ```text > tap riddle stone [Juniper's riddle stone] I answer every call and never speak first. What am I? (To answer: answer , or say to the riddle stone.) > answer a bird [Juniper's riddle stone] Not that one. Think of a valley. > say an echo, surely > say to the riddle stone: an echo [Juniper's riddle stone] An echo. That's the one. ``` Three things to notice. A verb's line that asks a question, from a toy that listens, is followed by a line saying how to answer. `answer` speaks to the thing that last asked. And "an echo, surely", said to nobody, was overheard and left alone. ## Matching words `includes?` finds a piece of text anywhere, which is sometimes too much: "bell" is in "bellow". Three other ways to look: - `(words (lower text))` gives the words, to check with `some` or a set: `((set (words (lower text))) "bell")`. - `(match text "i love *")` fits text to a pattern of words, where each `*` stands for one or more words. It answers the words the stars took, or `nil` when the text doesn't fit. Case and punctuation don't matter. - A pattern, `#"…"`, with `re-find`, for anything else. `match` makes a parrot that listens for a whole phrase: ```clojure (defobject parrot {:name "Parrot" :on-hear (fn [self who text] (when-let [m (match text "i love *")] (say self (str "You love " (first m) "! You love " (first m) "!"))))}) ``` ```text The parrot is ready. ``` ```text > say I love the roses [Juniper's parrot] You love the roses! You love the roses! > say I'd love some tea ``` "I'd love some tea" doesn't fit "i love *", so the parrot keeps quiet. ## Turning words round `reflect` turns a line round, as a mirror would: I and you, my and your change places, and the verbs follow: ```clojure (defobject mirror {:name "Mirror" :on-hear (fn [self who text] (when (:spoken-to self) (say self (str (reflect text) "?"))))}) ``` ```text The mirror is ready. ``` ```text > say to the mirror: I think you are lovely [Juniper's mirror] You think I am lovely? > say to the mirror: my garden needs rain [Juniper's mirror] your garden needs rain? ``` `reflect` keeps capitals where they were: "I" came back "You" at the start of a sentence, and "my", written small, came back "your". ## Knowing who spoke `who` is who spoke, as for a verb: `"@mira"` for a neighbour, `"@you"` for the toy's maker. A toy can greet its maker differently: ```clojure (defobject doormat {:name "Doormat" :on-hear (fn [self who text] (when (:spoken-to self) (say self (if (= who "@you") "Welcome home." (str "Wipe your feet, " who ".")))))}) ``` ```text The doormat is ready. ``` ```text > say hello to the doormat [Juniper's doormat] Wipe your feet, Mira. ``` ```text > say hello to the doormat [Your doormat] Welcome home. ``` ## Good company Words reach every listening toy at your home, so a toy that answers everything answers over every conversation. A few habits keep a listener pleasant: - Answer when spoken to (`(:spoken-to self)`), or to one word or phrase you chose. - Leave words said to someone else alone: `(when (nil? (:to self)) …)` answers only words said to nobody in particular, or to it. - Say one short line, not three. ## Try it 1. Make an owl that says "Hoo." when it hears the word "night", and only as a whole word. 2. Give the riddle stone a second riddle, chosen by a count in its state. 3. Make a toy that answers "Bless you." to "achoo", but only when nobody was spoken to. 4. Make a mirror that answers only its maker. ## Recap - `:on-hear (fn [self who text] …)` hears every line said at home. - `(:spoken-to self)` is true when the words were said to the toy; `(:to self)` names whom they were said to otherwise. - `includes?`, `words`, `match` and `reflect` are made for words heard. - `who` is `"@you"` for the maker. A good listener answers when spoken to. Next: a toy that keeps time. --- Before: [A first toy](https://plazmi.gg/book/first-toy.md). Next: [Keeping time](https://plazmi.gg/book/dawn.md). Contents: https://plazmi.gg/book.md