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core.clj
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;; trammel.clj -- Contracts programming library for Clojure
;; by Michael Fogus - <http://fogus.me/fun/trammel>
;; May 26, 2010
; Copyright (c) Michael Fogus, 2010. All rights reserved. The use
; and distribution terms for this software are covered by the Eclipse
; Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
; which can be found in the file COPYING the root of this
; distribution. By using this software in any fashion, you are
; agreeing to be bound by the terms of this license. You must not
; remove this notice, or any other, from this software.
(ns trammel.core
"The core contracts programming functions and macros for Trammel."
(:use [trammel.funcify :only (funcify)])
(:use trammel.factors))
;; HOF support
(defrecord HOC [argspec ctx])
(defmacro _ [& argspec]
`(HOC. (vec '~argspec) nil))
(comment
(_ even? number? => number?)
)
;; # base functions and macros
(defn- keys-apply
"Takes a function, a set of keys, and a map and applies the function to the map on the given keys.
A new map of the results of the function applied to the keyed entries is returned.
"
[f ks m]
{:pre [(or (fn? f) (keyword? f) (symbol? f))
(except (coll? ks) (map? ks))
(map? m)]
:post [(map? %)]}
(let [only (select-keys m ks)]
(zipmap (keys only)
(map f (vals only)))))
(defn- manip-map
"Takes a function, a set of keys, and a map and applies the function to the map on the given keys.
A modified version of the original map is returned with the results of the function applied to each
keyed entry.
"
[f ks m]
{:pre [(or (fn? f) (keyword? f) (symbol? f))
(except (coll? ks) (map? ks))
(map? m)]
:post [(map? %)
(= (keys %) (keys m))]}
(conj m (keys-apply f ks m)))
(defn- build-pre-post-map
"Takes a vector of the form `[pre ... => post ...]` and infers the expectations described
therein. The map that comes out will look like Clojure's default pre- anlein d post-conditions
map. If the argument is already a map then it's assumed that the default pre/post map is used and
as a result is used directly without manipulation.
"
[cnstr]
(if (vector? cnstr)
(let [[L M R] (partition-by #{'=>} cnstr)]
{:pre (when (not= L '(=>)) L)
:post (if (= L '(=>)) M R)})
cnstr))
(defn- build-constraints-map
"Takes the corresponding arglist and a vector of the contract expectations, the latter of which looks
like any of the following:
[(= 0 _)] or [number?] ;; only the pre-
[number? => number?] ;; a pre- and post-
[=> number?] ;; only a post-
[foo bar => baz] ;; 2 pre- and 1 post-
It then takes this form and builds a pre- and post-condition map of the form:
{:pre [(foo x) (bar x)]
:post [(baz %)]}
"
[args cnstr]
[args
(->> (build-pre-post-map cnstr)
(manip-map (partial funcify '[%]) [:post])
(manip-map (partial funcify args) [:pre]))])
(defn- build-contract
"Expects a seq representing an arity-based expectation of the form:
[[x] {:pre [(foo x)] :post [(bar %)]}]
It then uses this data to build another list reprsenting a specific arity body
for a higher-order function with attached pre- and post-conditions that directly
calls the function passed in:
([f x] {:pre [(foo x)] :post [(bar %)]} (f x))
However, the picture is slightly more compilcated than that because Clojure does
not have disparate pre-/post-conditions. Therefore, it's on me to provide a
slightly more crystaline picture of the condition failure when it occurs. As a
result the body of the contract is interwoven with `try`/`catch` blocks to catch
and examine the contents of `AssertionErrors` and based on context rethrow them
with more information. At the moment this information only takes the form of a
richer assertion message.
"
[n cnstr]
(let [[args pre-post-map] cnstr]
`(~(into '[f] args)
(let [ret# (try
((fn []
~(select-keys pre-post-map [:pre])
~(list* 'f (mapcat (fn [item]
(cond (symbol? item) [item]
(map? item) [(:as item)]
:else [item]))
args))))
(catch AssertionError pre#
(throw (AssertionError. (str "Pre-condition failure in " ~n "! " (.getMessage pre#))))))]
(try
((fn []
~(select-keys pre-post-map [:post])
ret#))
(catch AssertionError post#
(throw (AssertionError. (str "Post-condition failure in " ~n "! " (.getMessage post#))))))))))
(defmacro contract
"The base contract form returning a higher-order function that can then be partially
applied to an existing function to 'apply' a contract. Take for example a simple
contract that describes an expectation for a function that simply takes one or two
numbers and returns the double:
(def doubler-contract
(contract doubler
“Ensures that when given a number,
the result is doubled.”
[x] [number? => (= (* 2 x) %)]
[x y] [(every? number? [x y])
=>
(= (* 2 (+ x y)) %)]))
You can then partially apply this contract with an existing function:
(def doubler
(partial doubler-contract
#(* 2 %)))
(def bad-doubler
(partial doubler-contract
#(* 3 %)))
And then running these functions will be checked against the contract at runtime:
(doubler 2)
;=> 4
(bad-doubler 2)
; java.lang.AssertionError:
; Assert failed: (= (* 2 x) %)
Similar results would occur for the 2-arity versions of `doubler` and `bad-doubler`.
While it's fine to use `partial` directly, it's better to use the `with-constraints` function
found in this same library.
If you're so inclined, you can inspect the terms of the contract via its metadata, keyed on
the keyword `:constraints`.
"
[n docstring & constraints]
(if (not (string? docstring))
(throw (IllegalArgumentException. "Sorry, but contracts require docstrings"))
(let [raw-cnstr (partition 2 constraints)
arity-cnstr (for [[a c] raw-cnstr]
(build-constraints-map a c))
fn-arities (for [b arity-cnstr]
(build-contract docstring b))
body (list* 'fn n fn-arities)]
`(with-meta
~body
{:constraints (into {} '~arity-cnstr)
:docstring ~docstring}))))
(defn with-constraints
"A contract combinator.
Takes a target function and a number of contracts and returns a function with the contracts
applied to the original. This is the preferred way to apply a contract previously created
using `contract` as the use of `partial` may not work as implementation details change.
"
([f] f)
([f c] (partial c f))
([f c & more]
(apply with-constraints (with-constraints f c) more)))
(defmacro defcontract
"Convenience macro for defining a named contract. Equivalent to `(def fc (contract ...))`"
[name docstring & forms]
`(def ~name
(contract ~(symbol (str name "-impl")) ~docstring ~@forms)))
(defmacro defconstrainedfn
"Defines a function using the `contract` vector appearing after the arguments.
(defconstrainedfn sqr
[n] [number? (not= 0 n) => pos? number?]
(* n n))
Like the `contract` macro, multiple arity functions can be defined where each argument vector
is immediately followed by the relevent arity expectations. This macro will also detect
if a map is in that constraints position and use that instead under the assumption that
Clojure's `:pre`/`:post` map is used instead.
"
[name & body]
(let [mdata (if (string? (first body))
{:doc (first body)}
{})
body (if (:doc mdata)
(next body)
body)
body (if (vector? (first body))
(list body)
body)
body (for [[args cnstr & bd] body]
(list* args
(if (vector? cnstr)
(second (build-constraints-map args cnstr))
cnstr)
bd))]
`(defn ~name
~(str (:doc mdata))
~@body)))
(defmacro defconstrainedrecord
[name slots invariants & etc]
(let [fields (->> slots (partition 2) (map first) vec)
defaults (->> slots (partition 2) (map second))
ctor-name (symbol (str name \.))
factory-name (symbol (str "->" name))]
`(do
(let [t# (defrecord ~name ~fields ~@etc)]
(defn ~(symbol (str name \?)) [r#]
(= t# (type r#))))
(let [chk# (contract ~(symbol (str "chk-" name))
~(str "Invariant contract for " name)
[{:keys ~fields :as m#}] ~invariants)]
(defconstrainedfn ~factory-name
([] [] (with-meta
(~ctor-name ~@defaults)
{:contract chk#}))
([& {:keys ~fields :as kwargs# :or ~(apply hash-map slots)}]
~invariants
(with-meta
(-> (~ctor-name ~@defaults)
(merge kwargs#))
{:contract chk#}))))
~name)))
(defn- apply-contract
[f]
(if (:hooked (meta f))
f
(with-meta
(fn [m & args]
(if-let [contract (-> m meta :contract)]
((partial contract identity) (apply f m args))
(apply f m args)))
{:hooked true})))
(when *assert*
(alter-var-root (var assoc) apply-contract)
(alter-var-root (var dissoc) apply-contract)
(alter-var-root (var merge) apply-contract)
(alter-var-root (var merge-with) (fn [f] (let [mw (apply-contract f)] (fn [f & maps] (apply mw f maps)))))
(alter-var-root (var into) apply-contract)
(alter-var-root (var conj) apply-contract)
(alter-var-root (var assoc-in) apply-contract)
(alter-var-root (var update-in) apply-contract))
(defmacro defconstrainedtype
[name slots invariants & etc]
(let [fields (vec slots)
ctor-name (symbol (str name \.))
factory-name (symbol (str "->" name))]
`(do
(let [t# (deftype ~name ~fields ~@etc)]
(defn ~(symbol (str name \?)) [r#]
(= t# (type r#))))
(let [chk# (contract ~(symbol (str "chk-" name))
~(str "Invariant contract for " name)
[{:keys ~fields :as m#}] ~invariants)]
(defconstrainedfn ~factory-name
(~fields ~invariants
(~ctor-name ~@fields))))
~name)))
(comment
(use 'trammel.factors)
(defconstrainedfn leap-year?
[year],[number? pos? =>]
(and (= (mod year 4) 0)
(not (some #{(mod year 400)}
[100 200 300]))))
(leap-year? 111)
(let [year 0]
(in (mod year 400)
100 200 300))
(defconstrainedfn sqr
[n],[number? => pos? number?]
(* n n))
(sqr 0)
)