Writing a DSL

Let us make a calculator that would evaluate expressions like:

1 + 2

Dependencies:

[instaparse "1.4.10"]
[org.clojure/core.match "0.3.0-alpha5"]

Update required libs:

[instaparse.core :as insta]
[clojure.core.match :refer [match]]

Write the grammar as BNF (in the root of the project):

EXPR := NUMBER <ws*> OP <ws*> NUMBER

OP := ADD | SUB

ADD :=  "+"
SUB :=  "-"
NUMBER :=  #"[0-9]+"

ws := #'\s+'   (* whitespace *)

Parse the grammar:

(def parse (->> (System/getProperty "user.dir")
                (format "%s/grammar.bnf" )
                slurp
                insta/parser
                ))

Test it:

(parse "1 + 2")
;; [:EXPR [:NUMBER 1] [:OP [:ADD +]] [:NUMBER 3]]

Generate the ast:

(defn evaluate
  "Convert parse tree to abstract syntax tree"
  [parsed]
  (match parsed [:EXPR [:NUMBER l] [:OP op] [:NUMBER r]] (+ (read-string l) (read-string r) ) ;; hardcode the operator
         :else  {:error (format "Matching rules incomplete for: %s" parsed)}
         ))

Evaluate:

(->> "1 + 2"
     parse
     evaluate)

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• Tag: clojure