MIT licensed by Michael Snoyman
This version can be pinned in stack with:persistent-template-,1008

Module documentation for

There are no documented modules for this package.


This package was absorbed into persistent with the release.


Provides Template Haskell helpers for persistent. For more information, see the chapter in the Yesod book.

code organization

The TH.hs module contains code generators. persistent-template uses EntityDefs that it gets from the quasi-quoter. The quasi-quoter is in persistent Quasi.hs Similarly many of the types come from the persistent library

Development tips

To get a better idea of what code you’re generating, you can output the content of Template Haskell expressions to a file:

stack test persistent-template --ghc-options='-ddump-splices -ddump-to-file'

The output will be in the .stack-work directory. The exact path will depend on your specific setup, but if you search for files ending in .dump-splices you’ll find the output (find .stack-work -type f -name '*.dump-splices')

If you make changes to the generated code, it is highly recommended to compare the output with your changes to output from master (even better if this diff is included in your PR!). Seemingly small changes can have dramatic changes on the generated code.

For example, embedding an EntityDef in a function that was called for every field of that Entity made the number of generated lines O(N^2) for that function—very bad!


Unreleased changes

  • Decomposed HaskellName into ConstraintNameHS, EntityNameHS, FieldNameHS. Decomposed DBName into ConstraintNameDB, EntityNameDB, FieldNameDB respectively. #1174
  • #1214:
    • This package was asborbed into persistent itself.

  • #1145
    • Fix a bug where the SqlType for a shared primary key was being incorrectly set to SqlString instead of whatever the target primary key sql type was.
  • #1151
    • Automatically generate SymbolToField instances for datatypes, allowing OverloadedLabels to be used with the EntityField type.


  • Always use the “stock” strategy when deriving Show/Read for keys #1106

    • This fixes a regression from 2.8.0, which started using the newtype strategy when deriving Show/Read for keys
    • In practice, this means that from 2.8.0–, for the following schema:
     	name Text
     	anInt Int
     	Primary anInt
     	name Text

    PersonKey 1 would show as "SqlBackendKey {unSqlBackendKey = 1}" and CustomPrimaryKey 1 would show as "1"

    This was generally poor for debugging and logging, since all tables keys would print the same. For Persistent < 2.8.0 and >, they instead will show as:

    "PersonKey {unPersonKey = SqlBackendKey {unSqlBackendKey = 1}}" and "CustomPrimaryKey {unCustomPrimaryKey = 1}"

    This could be a breaking change if you have used Show on a key, wrote that string into some persistent storage like a database, and are trying to Read it back again later.

  • Add Lift instances for the cascade types. #1060
  • Use DeriveLift to implement all Lift instances. Among other benefits, this provides implementations of liftTyped on template-haskell-2.16 (GHC 8.10) or later. #1064

  • Require extensions in a more friendly manner. #1030
  • Specify a strategy for all deriving clauses, which avoids the -Wmissing-deriving-strategy warning introduced in GHC 8.8.2. #1030

  • Fix the mkPersist function to not require importing the classes explicitly. #1027

  • Fix the test-suite for persistent-template. #1023


  • Add fieldError to the export list of Database.Persist.TH #1008


  • Small optimization/code cleanup to generated Template Haskell code size, by slimming the implementation of to/fromPersistValue for Entities. #1014


  • Reduces the amount of code generated by Template Haskell. The amount of code generated for a certain function was O(N^2) with respect to the number of fields on a given Entity. This change shows dramatic improvements in benchmarks for compiling Persistent models. #
  • Drops support for GHC 8.0, so that DerivingStrategies can be used by persistent-template
  • persistent-template now requires DerivingStrategies, GeneralizedNewtypeDeriving, and StandaloneDeriving to be enabled in the file where Persistent entities are created
  • Fixes a long-standing issue where persistent-template would fail when DeriveAnyClass was enabled (See #578)
  • #1002


  • Remove an overlapping instance for Lift a. #998


  • Update module documentation for Database.Persist.TH to better describe the purpose of the module #968
  • Support template-haskell-2.15 #959


  • Expose the knot tying logic of parseReferences so that users can build migrations from independently define entities at runtime #932


  • Add the mkEntityDefList function to work around #902. #904


  • Depends on persistent-2.10.0 which provides the OnlyOneUniqueKey and AtLeastOneUniqueKey classes. Automatically generates instances for these classes based on how many unique keys the entity definition gets. This changes requires UndecidableInstances to be enabled on each module that generates entity definitions. #885
  • Removed deprecated sqlOnlySettings. Please use sqlSettings instead. #894



  • Slight improvement to the error message when a Persistent field can’t be parsed from database results


  • Exposed parseReferences to allow custom QuasiQuoters


  • Fix incorrect ToJSON/FromJSON instance generation for generic backends

Allow non-null self-references in a list

  • Allow composite Primary keys for tables that contain nullable fields.
  • Support foreign keys to non-integer ids

  • fix GHC 7.8 bug when a field name is “type”

  • fix a bad Eq instance /= definition for Key when mpsGenetric=True

  • workaround TH bug in GHC 7.10


  • read/write typeclass split


  • aeson 0.11
  • transformers 0.5


support http-api-data for url serialization

By default explicitly use Int64 for foreign key references. This avoids confusion on a 32 bit system.

Support foreign key references to composite primary keys

Support for monad-control 1.0