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Haskell: A Great Procedural Language

Haskell has garnered various catchphrases over the years, with some claiming it to be the best procedural language. This article delves into understanding why Haskell merits such a statement.

Side Effects as First Class Values

In Haskell, effectful computations are treated as first-class values, allowing them to be stored in variables or data structures. For instance, using the randomRIO function to generate random numbers results in encapsulating side effects rather than executing them immediately. This approach contrasts with other programming languages where side effects occur instantly.

De-Mystifying Do Blocks

Do blocks, often perceived as magical, can be broken down into two operators: *> (then) and >>= (bind). These operators allow for combining side effects sequentially or using results from one side effect in another, respectively.

Functions that Operate on Side Effects

Haskell offers several functions for handling side effects:

  • pure: Creates a side effect object with a specific value.
  • fmap: Transforms the result of a side effect using a function.
  • liftA2, liftA3, …: Combines results of multiple side effects using a function.
  • sequenceA: Executes a list of side effects, combining their results into a new side effect object.
  • traverse: Constructs a side effect object by applying a function to each element in a collection.
  • for: Similar to traverse but with arguments swapped for convenience.

Leaning into the First Classiness of Effects

Haskell’s ability to treat side effects as first-class values allows for advanced techniques like caching within procedural code. This flexibility illustrates Haskell’s prowess as a procedural language.

Things You Never Need to Care About

Several historic functions have been replaced with modern alternatives:

  • >> is now *>
  • return is now pure
  • map and liftM are now fmap
  • mapM is now traverse

Appendices

Avoiding Success and Uselessness: Haskell’s design philosophy prioritizes principles over widespread success, starting with strict function purity and gradually incorporating useful features.

Why fmap Maps Over Both Side Effects and Lists: The fmap function operates over functors, which include side effects, lists, and nullable values.

Foldable and Traversable: Differences in collection types influence how functions like sequenceA and traverse handle data structure shapes.

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