From 8ef91f03fac2bad20fb927fe2ca2782c66fdb513 Mon Sep 17 00:00:00 2001
From: Akshay Nair <phenax5@gmail.com>
Date: Thu, 6 Jan 2022 23:41:32 +0530
Subject: feat: adds more parser combinators

---
 src/index.ts  | 12 +++++++++++-
 src/parser.ts | 24 +++++++++++++++++++++++-
 2 files changed, 34 insertions(+), 2 deletions(-)

(limited to 'src')

diff --git a/src/index.ts b/src/index.ts
index d236473..582f9be 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,5 +1,5 @@
 import { constant, flow, identity, pipe } from 'fp-ts/function'
-import { mapTo, symbol } from './parser'
+import { andThen, delimited, mapTo, optional, pair, symbol } from './parser'
 
 export const start = mapTo(symbol('^'), constant({ tag: 'Start' } as Expr))
 export const end = mapTo(symbol('$'), constant({ tag: 'End' } as Expr))
@@ -27,6 +27,16 @@ type Expr =
   | { tag: 'Truthy' }
   | { tag: 'Falsey' }
 
+export const expressionP = symbol('fuck')
+
+export const parser = pair(
+  pair(
+    optional(start),
+    expressionP,
+  ),
+  optional(end),
+)
+
 /*
 
 ^ $ => start and end of list
diff --git a/src/parser.ts b/src/parser.ts
index f8dcd44..5ccba43 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -1,5 +1,6 @@
 import { flow, pipe } from 'fp-ts/function'
-import { Either, left, right, map, chain, orElse } from 'fp-ts/Either'
+import { Either, left, right, map, chain, orElse, fold } from 'fp-ts/Either'
+import { none, some, Option } from 'fp-ts/lib/Option'
 
 export type char = string
 
@@ -117,3 +118,24 @@ export const mapTo = <I, R>(p: Parser<I>, f: (p: I) => R): Parser<R> =>
     p,
     map(([v, inp]) => [f(v), inp])
   )
+
+export const andThen = <I, R>(f: (p: I) => Parser<R>) => (p: Parser<I>): Parser<R> =>
+  flow(
+    p,
+    chain(([v, inp]) => f(v)(inp)),
+  )
+
+export const optional = <T>(p: Parser<T>): Parser<Option<T>> =>
+  flow(
+    p,
+    fold(
+      ([_, inp]) => right([none, inp]),
+      ([v, inp]) => right([some(v), inp])
+    )
+  )
+
+export const pair = <A, B>(a: Parser<A>, b: Parser<B>): Parser<[A, B]> => pipe(
+  a,
+  andThen(ra => mapTo(b, rb => [ra, rb]))
+)
+
-- 
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