This version can be pinned in stack with:strict-0.5@sha256:bd57d7b3655951dfaa7d1e1374d7352dfe83f82a9c98309bf3a6587ef8cbd87d,4120

This package provides strict versions of some standard Haskell data types (pairs, Maybe and Either). It also contains strict IO operations.

It is common knowledge that lazy datastructures can lead to space-leaks. This problem is particularly prominent, when using lazy datastructures to store the state of a long-running application in memory. One common solution to this problem is to use seq and its variants in every piece of code that updates your state. However a much easier solution is to use fully strict types to store such state values. By "fully strict types" we mean types for whose values it holds that, if they are in weak-head normal form, then they are also in normal form. Intuitively, this means that values of fully strict types cannot contain unevaluated thunks.

To define a fully strict datatype, one typically uses the following recipe.

  1. Make all fields of every constructor strict; i.e., add a bang to all fields.

  2. Use only strict types for the fields of the constructors.

The second requirement is problematic as it rules out the use of the standard Haskell Maybe, Either, and pair types. This library solves this problem by providing strict variants of these types and their corresponding standard support functions and type-class instances.

Note that this library does currently not provide fully strict lists. They can be added if they are really required. However, in many cases one probably wants to use unboxed or strict boxed vectors from the vector library ( instead of strict lists. Moreover, instead of Strings one probably wants to use strict Text values from the text library (

This library comes with batteries included; i.e., mirror functions and instances of the lazy versions in base. It also includes instances for type-classes from the deepseq, binary, and hashable packages.



  • Allow bytestring-0.11
  • Remove duplicate semigroups dependency


  • Add instances for type-classes in current base, binary, deepseq and hashable
  • Add combinators mirroring Data.Maybe, Data.Either and Data.Tuple
  • Add Strict lazy strict type-class with toStrict / toLazy conversion functions
  • Modules are explicitly marked Safe or Trustworthy
  • Add strict These
  • :!: (Pair constructor) is non-associative