As a follow up to my previous question , I am trying to implement a simple pattern matching in Clojure.
I would like something like the following:
(match target
[ub] expr1 ; ub should be bound to actual value in expr1
['< ub] expr2 ; match the literal less-than symbol
; and ub should be bound to actual value in expr2
[lb ub] expr3 ; lb and ub should be bound to actual values in expr3
:else expr4 ; default case if none match
)
Usage:
(match [< 5.0] ...)
should arrange to execute expr2
at runtime.
I would like to write a macro, but I am not sure of the expansion.
I am considering having each case-and-clause expand to a let
with bindings to internal variables and checks that the literal symbols ( '<
) actually matches the pattern. Maybe for the second pattern ( ['< ub]
):
(let [[sym1 ub] pattern]
(if (= '< sym1)
expr1)
Do I need to use (gensym)
for the bindings? How?
Bigger picture:
(range-case target
[0.0 < 1.0] :greatly-disagree
[< 2.0] :disagree
[< 3.0] :neutral
[< 4.0] :agree
[5.0] :strongly-agree
42 :the-answer
:else :do-not-care)
I am trying to match the [...]
patterns and convert them to the following:
[ub] (if previous-ub `(and (<= ~previous-ub ~target) (<= ~target ~ub))
`(< ~target ~ub))
['< ub] (if previous-ub `(and (<= ~previous-ub ~target) (< ~target ~ub))
`(< ~target ~ub))
[lb ub] `(and (<= ~lb ~target) (<= ~target ~ub))
['< lb ub] `(and (< ~lb ~target) (<= ~target ~ub))
[lb '< ub] `(and (<= ~lb ~target) (< ~target ~ub))
['< lb '< ub] `(and (< ~lb ~target) (< ~target ~ub))
I have a cond
that checks that the case part is a vector. This pattern match should occur inside that case.
My first idea was basically the same: Bind stuff to internal locals and test on their contents in a big and
. For literals the value is bound to a generated local; symbols are used directly in the binding.
I also added a check that the spec vector matches the length of the target vector. Otherwise you can't have [ub]
as well as [lb ub]
since neither contains a check which could fail. So always the first would be selected.
Here is the code:
(defn make-clause
[expr-g [spec expr & more :as clause]]
(when (seq clause)
(let [tests-and-bindings (map (fn [x]
(if-not (symbol? x)
(let [x-g (gensym "x")]
[`(= ~x ~x-g) x-g])
[nil x]))
spec)
tests (keep first tests-and-bindings)
bindings (map second tests-and-bindings)]
`(let [[~@bindings] ~expr-g]
(if (and (= (count ~expr-g) ~(count spec)) ~@tests)
~expr
~(make-clause expr-g more))))))
(defmacro match
[expr & clauses]
(let [expr-g (gensym "expr")]
`(let ~[expr-g expr]
~(make-clause expr-g clauses))))
And an example expansion. I didn't use syntax-quote in the example to reduce the noise in the expansion, but you should get the idea.
(let [expr98 [(quote <) 3.0]]
(let [[ub] expr98]
(if (and (= (count expr98) 1))
(if previous-ub
(and (<= previous-ub target) (<= target ub))
(< target ub))
(let [[x99 ub] expr98]
(if (and (= (count expr98) 2) (= (quote <) x99))
(if previous-ub
(and (<= previous-ub target) (< target ub))
(< target ub))
(let [[lb ub] expr98]
(if (and (= (count expr98) 2))
(and (<= lb target) (<= target ub))
nil)))))))
The invokation was:
(match ['< 3.0]
[ub] (if previous-ub
(and (<= previous-ub target) (<= target ub))
(< target ub))
['< ub] (if previous-ub
(and (<= previous-ub target) (< target ub))
(< target ub))
[lb ub] (and (<= lb target) (<= target ub))))
Hope that helps you get started.
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.