hasql-dynamic-statements: Hasql extension for dynamic construction of statements

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Library extending Hasql with a composable API for building SQL statements dynamically.

The core abstraction is Snippet, which allows you to construct SQL statements with parameters injected in-place, automatically handling placeholders and encoder matching. This is particularly useful when the structure of your SQL depends on runtime parameters.

Key features:

Composable SQL snippets using Semigroup and Monoid instances
Automatic placeholder generation and parameter encoding
Type-safe parameter injection with implicit encoder derivation
Protection against common SQL construction bugs
Support for conditional SQL structure based on parameters

Example:

selectSubstring :: Text -> Maybe Int32 -> Maybe Int32 -> Snippet
selectSubstring string from to =
  "select substring(" <> param string <>
  foldMap (\x -> " from " <> param x) from <>
  foldMap (\x -> " for " <> param x) to <>
  ")"

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Hasql
- DynamicStatements
  - Hasql.DynamicStatements.Snippet

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hasql-dynamic-statements-0.5.0.1.tar.gz [browse] (Cabal source package)
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Versions [RSS]	0.1, 0.1.0.1, 0.1.0.2, 0.2, 0.2.0.1, 0.2.0.2, 0.2.0.3, 0.3, 0.3.1, 0.3.1.1, 0.3.1.2, 0.3.1.3, 0.3.1.4, 0.3.1.5, 0.3.1.6, 0.3.1.7, 0.3.1.8, 0.4, 0.5, 0.5.0.1
Change log	CHANGELOG.md
Dependencies	base (>=4.14 && <5), bytestring (>=0.10 && <0.13), containers (>=0.6 && <0.8), hasql (>=1.10 && <1.11), hasql-implicits (>=0.2.0.2 && <0.3), text (>=1 && <3), text-builder (>=1.0 && <1.1) [details]
License	MIT
Copyright	(c) 2019, Nikita Volkov
Author	Nikita Volkov <nikita.y.volkov@mail.ru>
Maintainer	Nikita Volkov <nikita.y.volkov@mail.ru>
Uploaded	by NikitaVolkov at 2026-01-14T09:47:00Z
Home page	https://github.com/nikita-volkov/hasql-dynamic-statements
Bug tracker	https://github.com/nikita-volkov/hasql-dynamic-statements/issues
Source repo	head: git clone https://github.com/nikita-volkov/hasql-dynamic-statements
Distributions	Arch:0.3.1.1, LTSHaskell:0.3.1.8, NixOS:0.3.1.8, Stackage:0.3.1.8
Reverse Dependencies	2 direct, 2 indirect [details]
Downloads	5456 total (79 in the last 30 days)
Rating	(no votes yet) [estimated by Bayesian average]
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Status	Docs uploaded by user Build status unknown [no reports yet]

Readme for hasql-dynamic-statements-0.5.0.1

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hasql-dynamic-statements

Hasql extension for composable, dynamic construction of SQL statements.

Overview

This library provides a Snippet API that simplifies building SQL statements where the structure depends on runtime parameters. It abstracts over placeholder management and encoder matching, eliminating boilerplate and reducing bugs.

Key Features

Composable: Build statements using Semigroup/Monoid operations
Type-safe: Automatic parameter encoding with type-driven encoder derivation
Dynamic: Construct SQL conditionally based on parameters
Clean API: No manual placeholder numbering or encoder alignment

Quick Example

import Hasql.DynamicStatements.Snippet

-- Build a dynamic substring query
selectSubstring :: Text -> Maybe Int32 -> Maybe Int32 -> Snippet
selectSubstring string from to =
  "select substring(" <> param string <>
  foldMap (\x -> " from " <> param x) from <>
  foldMap (\x -> " for " <> param x) to <>
  ")"

-- Execute in a session
result <- toSession (selectSubstring "hello" (Just 2) Nothing) 
  (singleRow $ column $ nonNullable text)

Without Snippet API

Compare the above to manual construction:

selectSubstring' :: Text -> Maybe Int32 -> Maybe Int32 -> Statement () Text
selectSubstring' string from to = 
  let sql = case (from, to) of
        (Just _, Just _)  -> "select substring($1 from $2 to $3)"
        (Just _, Nothing) -> "select substring($1 from $2)"
        (Nothing, Just _) -> "select substring($1 to $2)"
        (Nothing, Nothing) -> "select substring($1)"
      encoder = 
        Encoders.param (string >$ Encoders.text) <>
        foldMap (\x -> Encoders.param (x >$ Encoders.int8)) from <>
        foldMap (\x -> Encoders.param (x >$ Encoders.int8)) to
      decoder = singleRow $ column $ nonNullable text
  in Statement.preparable sql encoder decoder

The Snippet API eliminates placeholder numbering, pattern matching on parameter presence, and manual encoder sequencing.