Preferences

Like normal Clojure multimethods, Methodical multimethods support specifying preferences for one dispatch value over another so it knows which method to use in situation where neither of two dispatch values is considered more specific than another (e.g., neither one isa? the other).

Adding Preferences

[[methodical.core/prefer-method]] works just like clojure.core/prefer-method, with one important difference: it is non-destructive. [[methodical.core/prefer-method]] will return a copy of the original multimethod with an updated preferences table.

To destructively add a preference to a methodical multimethod, use [[methodical.core/prefer-method!]] and the var you'd like to update:

;;; add a preference of :x over y
(m/prefer-method! #'my-multimethod :x :y)

You can also non-destructively replace the entire preferences table with a new one using the low-level method [[methodical.core/with-prefers]], but you should use caution in doing so. [[methodical.core/prefer-method]] will check to make sure any preferences you add are valid (e.g., no conflicts between preferences), but [[methodical.core/with-prefers]] will not.

[[methodical.core/with-prefers!]] is a destructive version of [[methodical.core/with-prefers]] for use on multimethod vars.

Inspecting Preferences

[[methodical.core/prefers]] works just like clojure.core/prefers and can be used to get the preferences map for a multimethod.

Removing Preferences

Unlike normal Clojure multimethods, Methodical also supports removing preferences. [[methodical.core/unprefer-method]] undoes a preference added by [[methodical.core/prefer-method]]. Like [[methodical.core/prefer-method]], this is a non-destructive function that returns a copy of the original multimethod with an updated preferences map; you can use [[methodical.core/unprefer-method!]] to destructively update a multimethod var.

[[methodical.core/remove-all-preferences]] non-destructively removes all preferences for a multimethod by replacing its preferences map with an empty one. [[methodical.core/remove-all-preferences!]] is a destructive version for doing the same operation on a multimethod var.