Parser Combinators

# Parser Combinators Source: [https://t3.at/blog/2026-10-17-parser-combinators/](https://t3.at/blog/2026-10-17-parser-combinators/) `"Hello, world\!"``\{ greeting: "Hello", who: "world" \}` The string "Hello, world\!" parses into an object with greeting "Hello", who "world"\. `type Parser<T\> = \(input: string\) =\> \[T, string\]` `\["pIntegernumber10, "string 20 30"\]` The type Parser of T is a function from an input string to a pair of a T and the rest of the string\. For example, pInteger cuts "10 20 30" after"10", and returns the number 10 and the rest of the string," 20 30"\. `pHello``\["Hello", " 100"\]` `pIgnoreWhitespace``\[" null", "100"\]` `pInteger``\["100", ""\]` "Hello 100" parsed by pHello, then pIgnoreWhitespace, then pInteger, each taking the rest the one before left over: pHello returns "Hello" and " 100"\. pIgnoreWhitespace returns " " and "100"\. pInteger returns "100" and ""\. `pHello100``\["Hello 100\["Hello", null, 100\]", ""\]` "Hello 100" parsed by pHello100, each taking the rest the one before left over: pHello100 returns "Hello 100" and ""\. `pSeq\( pHello, pIgnoreWhitespace, pInteger \)``Parser<\[ string, null, number \]\>` pSeq given pHello, pIgnoreWhitespace, pInteger is a Parser of the tuple \[string, null, number\]\. --- `const pHello100 = pSeq\(\.\.\.\)``const result = pHello100\("Hello 100"\)` `pHello100``\["Hello 100\["Hello", null, 100\]", ""\]` "Hello 100" parsed by pHello100, each taking the rest the one before left over: pHello100 returns "Hello 100" and ""\. `pGreeting``\["Hello 100\["Hello", null, 100\]\{ greeting: "Hello", who: 100 \}pMap", ""\]` "Hello 100" parsed by pGreeting, each taking the rest the one before left over: pGreeting returns "Hello 100" and ""\. `pSeq\(pNumber, pSemicolon\)``\["10;\[10, ";"\]", "20;30;"\]` `pSeq\(pNumber, pSemicolon\)``\["20;\[20, ";"\]", "30;"\]` `pSeq\(pNumber, pSemicolon\)``\["30;\[30, ";"\]", ""\]` "10;20;30;" parsed by pSeq\(pNumber, pSemicolon\), then pSeq\(pNumber, pSemicolon\), then pSeq\(pNumber, pSemicolon\), each taking the rest the one before left over: pSeq\(pNumber, pSemicolon\) returns "10;" and "20;30;"\. pSeq\(pNumber, pSemicolon\) returns "20;" and "30;"\. pSeq\(pNumber, pSemicolon\) returns "30;" and ""\. `PING 1\.1\.1\.1 \(1\.1\.1\.1\) 56\(84\) bytes of data\. 64 bytes from 1\.1\.1\.1: icmp\_seq=1 ttl=52 time=389 ms 64 bytes from 1\.1\.1\.1: icmp\_seq=2 ttl=52 time=68\.7 ms 64 bytes from 1\.1\.1\.1: icmp\_seq=3 ttl=52 time=47\.1 ms 64 bytes from 1\.1\.1\.1: icmp\_seq=4 ttl=52 time=48\.6 ms 64 bytes from 1\.1\.1\.1: icmp\_seq=5 ttl=52 time=85\.8 ms 64 bytes from 1\.1\.1\.1: icmp\_seq=6 ttl=52 time=84\.8 ms 64 bytes from 1\.1\.1\.1: icmp\_seq=7 ttl=52 time=87\.4 ms \-\-\- 1\.1\.1\.1 ping statistics \-\-\- 8 packets transmitted, 7 received, 12\.5% packet loss, time 7008ms rtt min/avg/max/mdev = 47\.129/115\.880/388\.750/112\.515 ms` `\{ header: \{ targetIp: "1\.1\.1\.1", resolvedIp: "1\.1\.1\.1", icmpPayloadSize: 56, totalPacketSize: 84 \}, packets: \[ \{ byteCount: 64, from: "1\.1\.1\.1", seq: 1, ttl: 52, time: 389 \}, \{ byteCount: 64, from: "1\.1\.1\.1", seq: 2, ttl: 52, time: 68\.7 \}, \{ byteCount: 64, from: "1\.1\.1\.1", seq: 3, ttl: 52, time: 47\.1 \}, \{ byteCount: 64, from: "1\.1\.1\.1", seq: 4, ttl: 52, time: 48\.6 \}, \{ byteCount: 64, from: "1\.1\.1\.1", seq: 5, ttl: 52, time: 85\.8 \}, \{ byteCount: 64, from: "1\.1\.1\.1", seq: 6, ttl: 52, time: 84\.8 \}, \{ byteCount: 64, from: "1\.1\.1\.1", seq: 7, ttl: 52, time: 87\.4 \}, \], statistics: \{ packetsTransmitted: 8, packetsReceived: 7, packetLoss: 12\.5, time: 7008, rtt: \{ min: 47\.129, avg: 115\.88, max: 388\.75, mdev: 112\.515 \}, \}, \}` Fold`const result = list\.reduce\(\(acc, n\) =\> acc \+ n\)``\["Hello", " ", 100\] → "Hello 100"` --- Unfold`const result = pSeq\(\.\.\.\)\("Hello 100"\)``"Hello 100" → \["Hello", " ", 100\]`