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Comment by frank2 | Hacker News Reader

Haskell, which uses a HM type system, has a type called Dynamic, which is described as follows:

>Finally, dynamically typed programming in Haskell made easy! Tired of making data types in your Haskell programs just to read and manipulate basic JSON/CSV files? Tired of writing imports? Use dynamic, dynamically typed programming for Haskell! [1]

If using an HM type system is really as easy and as fluid as you suggest, what purpose would be served by a type such as Haskell's Dynamic?

Nor is Dynamic a panacea: to use it requires a lot of extra typing (er, extra keypresses) IIRC, and

>A Dynamic may only represent a monomorphic value; an attempt to create a value of type Dynamic from a polymorphically-typed expression will result in an ambiguity error (see toDyn). [2]

1: https://hackage.haskell.org/package/dynamic

2: https://hackage.haskell.org/package/base-4.14.0.0/docs/Data-...

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It's probably worth noting that the majority of the development on that library was done on April 1st 2019.

The relevant file of source code has, "Copyright (c) The University of Glasgow 2001", which jibes with my memory of having used Dynamic in GHC in 2004 or earlier:

https://hackage.haskell.org/package/base-4.14.0.0/docs/src/D...

Also notice that Dynamic is in the Haskell Package Repository's "base" package, described as containing "Basic libraries", not part of an ancillary area for user-contributed libraries:

https://hackage.haskell.org/package/base-4.14.0.0

There exist type systems of which great grandparent is an accurate description (the most well-known of which is probably that used by Typescript) but the type system of, e.g., Haskell, Ocaml or Rust is not it.

(Now I am getting curious what specific programming language's type system great grandparent had in mind.)

I assume aweinstock was referring to the first link you posted:

https://hackage.haskell.org/package/dynamic

Which is an entirely different package, and the relevant types are entirely different. Only the name is the same. The `dynamic` package does indeed appear to be an april fools joke.

The Dynamic type in base is about runtime reflection, which is occasionally useful but not something that comes up often and not really about rapid prototyping.

I didn't notice they were different packages.

>The Dynamic type in base is about runtime reflection, which is occasionally useful but not something that comes up often

It is not something that comes up often, but since I am used to languages like Javascript or Scheme, when it does come up, figuring out how to do it in Haskell, e.g., satisfying the monomorphic restriction, has occasionally diverted my focus away from the problem to be solved for hours while I learn more about Haskell's type system.

But yeah, what I just wrote is in no way inconsistent with the comment I initially replied to, namely,

https://news.ycombinator.com/item?id=23795687

so I am going to admit that my comments in this thread have not exactly improved the level of discourse.

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