clojure.spec custom generator for Java objects - clojure

Question

I just saw one of Rich's talks on clojure.spec, and really want to give it a try on my project. I'm writing a series of tools for parsing C code using the eclipse CDT library, and I would like to spec that my functions accept and emit AST objects.
I think a very basic spec could be written for a function that takes the root of an AST and emits all the tree's leaves like this:
(import '(org.eclipse.cdt.core.dom.ast IASTNode))
(require '[clojure.spec :as s])
(defn ast-node? [node] (instance? IASTNode node))
(s/def ::ast-node ast-node?)
(s/fdef leaves :args ::ast-node :ret (s/coll-of ::ast-node))
However when I try to exercise the code (s/exercise leaves) I get an error:
Unable to construct gen at: [] for:
xxx.x$leaves#xxx
#:clojure.spec{:path [], :form #function[xxx.xxx/leaves], :failure :no-gen}
How can I write a custom generator for Java objects to fully spec and exercise my code?

Answer 1

You can attach a custom generator to a spec using s/with-gen. You'll need to write a generator that produces all the node variants that you need. You might find it easier to write one generator per node type and then combine them, either with s/or or possibly by using something like s/multi-spec instead (which would make this open to extension).
An example of writing a generator that produces a Java object would be something like this:
(s/def ::date
(s/with-gen #(instance? java.util.Date %)
(fn [] (gen/fmap #(java.util.Date. %) (s/gen pos-int?)))))
fmap takes a function and applies that to each result from the generator you give it. If you have a Java object with a constructor that takes multiple values, you can use a source generator like (s/gen (s/tuple int? string? int?)).

Answer 2

For completeness, here's my code after applying Alex's answer to spec a "LiteralExpression" AST node:
(ns atom-finder.ast-spec
(:import [org.eclipse.cdt.internal.core.dom.parser.cpp CPPASTLiteralExpression])
(:require [clojure.spec :as s]
[clojure.spec.gen :as gen]))
(def gen-literal-expression-args
(gen/one-of
[
(gen/tuple (s/gen #{CPPASTLiteralExpression/lk_char_constant})
(gen/char-ascii))
(gen/tuple (s/gen #{CPPASTLiteralExpression/lk_float_constant})
(gen/double))
(gen/tuple (s/gen #{CPPASTLiteralExpression/lk_integer_constant})
(s/gen (s/int-in -2147483648 2147483647)))
(gen/tuple (s/gen #{CPPASTLiteralExpression/lk_string_literal})
(gen/string))]))
(def gen-literal-expression
(gen/fmap
(fn [[type val]]
(CPPASTLiteralExpression. type (.toCharArray (str val))))
gen-literal-expression-args))
(s/def ::literal-expression
(s/with-gen
(partial instance? CPPASTLiteralExpression)
(fn [] gen-literal-expression)))
(s/exercise :atom-finder.ast-spec/literal-expression 10