pa-field-parser: “Vertical” parsing of values

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Dependencies aeson, aeson-better-errors, attoparsec, base (<5), case-insensitive, containers, pa-error-tree, pa-prelude, scientific, semigroupoids, template-haskell, text, time [details]
License BSD-3-Clause
Copyright 2023 Possehl Analytics GmbH
Maintainer Philip Patsch <>
Category Data, Possehl-Analytics
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Uploaded by Profpatsch at 2023-07-28T09:09:42Z
Distributions NixOS:
Downloads 81 total (7 in the last 30 days)
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Status Docs available [build log]
Last success reported on 2023-07-28 [all 1 reports]

Readme for pa-field-parser-

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A small library for “vertical parsing” of values.

Traditional parsers are “horizontal” parsers, they take a flat list of tokens and produce structure:

[  token1 token2 token3 token4 token5 ]
----> horizontal parser


- token1
`-- token2
`-- token3
  `-- token 4 -- token 5

A FieldParser is a “vertical” parser. Once you have some low-level type in hand, usually you want to do some more checks, to “upgrade” it so to say:

    | signedDecimal
    | utf8

As a FieldParser, this would look like:

utf8 :: FieldParser ByteString Text
signedDecimal :: FieldParser Text Integer

(utf8 >>> signedDecimal) :: FieldParser ByteString Integer

>>> is from Control.Category, but Profunctor is also available to map over the left and right arguments.

When run, this produces either a value or a helpful error message.

They can be freely combined with other libraries, and act as a nice adapter between them. For example, the JSON-related functions integrate with aeson-better-errors and any FieldParser Value a can be converted to a FromJSON instance for aeson. attoparsec is also available to easily turn bytes or text parsers into FieldParsers.

You can use this library as-is, but the design is easily adaptable to your codebase, the idea is very simple. Feel free to copy and paste what you need.