From db97b1a9c42fe945ca37256f5ee18d52c6aa32b4 Mon Sep 17 00:00:00 2001
From: Akshay Nair <phenax5@gmail.com>
Date: Thu, 10 Aug 2023 18:18:46 +0530
Subject: feat: adds parser for simple expressions

---
 src/parse-expr.ts | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

(limited to 'src/parse-expr.ts')

diff --git a/src/parse-expr.ts b/src/parse-expr.ts
index 53de485..dbad1a1 100644
--- a/src/parse-expr.ts
+++ b/src/parse-expr.ts
@@ -1,11 +1,38 @@
-import { Enum, constructors } from "./utils/adt";
+import { Enum, constructors, match } from './utils/adt';
+import * as P from './utils/parser-comb'
 
 export type Expr = Enum<{
   Call: { name: string, args: Expr[] }
   Var: { name: string, defaultValue: Expr }
   Identifier: string
   LiteralString: string
-}>;
+}>
 
-export const Expr = constructors<Expr>();
+export const Expr = constructors<Expr>()
+
+const whitespace = P.regex(/\s*/)
+const consumeWhitespace = <A>(p: P.Parser<A>): P.Parser<A> => P.between(whitespace, p, whitespace)
+
+const identifierParser = P.regex(/^[a-z][a-z0-9]+/i)
+const identifierExprParser = P.map(identifierParser, ident => Expr.Identifier(ident))
+
+const callParser = P.map(P.zip2(
+  consumeWhitespace(identifierParser),
+  P.between(P.string('('), consumeWhitespace(identifierParser), P.string(')')),
+), ([name, rest]) => Expr.Call({ name, args: [Expr.Identifier(rest)] }))
+
+const exprParser: P.Parser<Expr> = P.or([
+  callParser,
+  identifierExprParser,
+])
+
+export const parse = (input: string): Expr => {
+  const res = exprParser(input.trim());
+  return match(res, {
+    Ok: ({ value }) => value,
+    Err: ({ error, input }) => {
+      throw new Error(`${error}.\n Left input: ${input.slice(0, 20)}...`)
+    },
+  })
+}
 
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