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[CLJ-825] Protocol implementation inconsistencies when overloading arity Created: 08/Aug/11  Updated: 10/Jun/14

Status: Open
Project: Clojure
Component/s: None
Affects Version/s: Release 1.2, Release 1.3, Release 1.4, Release 1.5
Fix Version/s: None

Type: Enhancement Priority: Major
Reporter: Carl Lerche Assignee: Unassigned
Resolution: Unresolved Votes: 4
Labels: protocols


Attachments: Text File clj-825-1.patch     File scribbles.clj    
Patch: Code and Test
Approval: Triaged


The forms required for implementing arity-overloaded protocol methods are inconsistent between the "extend-*" macros and "defrecord".

The "extend" family of macros requires overloaded method definitions to follow the form used by defn:

(method ([arg1] ...) ([arg1 arg2] ...))

However, "defrecord" requires implementations to be defined separately:

(method [arg1] ...)
(method [arg1 arg2] ...)

Furthermore, the error modes if you get it wrong are unhelpful.

If you use the "defrecord" form with "extend-*", it evals successfully, but later definitions silently overwrite lexically previous definitions.

If you use the "extend-*" form with "defrecord", it gives a cryptic error about "unsupported binding form" on the body of the method.

This is not the same issue as CLJ-1056: That pertains to the syntax for declaring a protocol, this problem is with the syntax for implementing a protocol.

(defprotocol MyProtocol
    [this arg]
    [this arg optional-arg]))

(extend-protocol MyProtocol
    ([this arg] :one-arg)
    ([this arg optional-arg] :two-args)))

;; BAD! Blows up with "Unsupported binding form: :one-arg"
(defrecord MyRecord []
    ([this arg] :one-arg)
    ([this arg optional-arg] :two-args)))

;; Works...
(defrecord MyRecord []
  (mymethod [this arg] :one-arg)
  (mymethod [this arg optional-arg] :two-args))

;; Evals...
(extend-protocol MyProtocol
  (mymethod [this arg] :one-arg)
  (mymethod [this arg optional-arg] :two-args))

;; But then... Error! "Wrong number of args"
(mymethod :obj :arg)

;; 2-arg version is invokable...
(mymethod :obj :arg1 :arg2)

Comment by Paavo Parkkinen [ 17/Nov/13 6:02 AM ]

Attached a patch for this.

For defrecord, I check which style is used for defining methods, and transform into the original style if the new style is used. For the check I do what I believe defn does, which is (vector? (first fdecl)).

For extend-*, I skip the checking, and just transform everything into the same format.

Tests included for both.

All tests pass.

Comment by Rich Hickey [ 10/Jun/14 11:00 AM ]

What the proposal?

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