My starting point for ghostwheel type checking

ghostwheel-config.cljc

(ns bookserver.io
  #:ghostwheel.core{:trace       true
                    :check       false
                    :num-tests   0
                    ; oustrument to check types at run-time for all functions
                    ; in this namespace
                    :outstrument true}

  (:require [clojure.spec.alpha :as s]
            [ghostwheel.core :as g
             :refer [>defn >defn- >fdef => | <- ?]]))

; human readable spec errors, from expound library
(set! s/*explain-out* expound.alpha/printer)

; example function
(>defn create-hiccup-table-row
  " take a vector of values [0 2 2 ...], turn them into hiccup "
  [row]
  [vector? =>
   ; vector of hiccup: [:tr [:td 0] [:td 2] ...]
   vector?]
  (->>  row
        (map #(conj [:td] %))
        (apply conj [:tr])))

; force tracing, overwriting ns level defaults
(>defn ^::g/trace linkify-embed-images-desc
  [card]
  [map? => vector?]
  nil)

; @genekim Oh, `::g/trace` is just a regular metadata key like everything else and it's set to 0 by default. 
; So you just have to attach it to the function like you would any other function metadata in Clojure, so 
; either `(>defn ^{::g/trace 0} fn-name ...)` or `(>defn fn-name {::g/trace 0} ...}`

(>defn ^{::g/trace 0} linkify-embed-images-desc
  [card]
  [map? => vector?]
  nil)

; super handy spec is sequential?, which accepts seqs, lists, vectors
; (s/nulable sequential?) allows nil values, too.
(>defn foo
  [coll]
  [(s/nllable sequential?) => sequential?)

Once you've set :outstrument true, in a REPL, you can confirm that malformed function args will generate a spec error. Using expound makes errors more human-readable, as well.

I used to do things like this, to confirm the correct types, which was super-helpful, but sometimes cumbersome to write:

(defn index-of [coll value]
  {:pre [(s/valid? coll? coll)]}
  (some (fn [[idx item]] (if (= value item) idx))
        ; [x y z] -> [[0 x] [1 y] [2 z]]
        (map-indexed vector coll)))

I find ghostwheel syntax to be much more pleasing and easier to write.

As I have to read or modify code I wrote before, I find myself adding ghostwheel-style typing information, which makes me very happy. Thank you @gnl!

In Java 10, you must require [expound "0.7.1"], otherwise you'll get an exception when you (set!).