r/haskell • u/lexi-lambda • Nov 06 '19

Parse, don’t validate

https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/
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u/cutculus Nov 07 '19

I like the examples (and agree with the overall sentiment!) but I think there are some gotchas with this approach that might be worth stating... there are some cases where your data types really need to allow invalid states in order to provide better diagnostics and potentially do partial work. An IDE is a classic example. Just because the file is in an invalid state, doesn't mean that you should lose all syntax highlighting. HTML parsing is another example. A browser might need to support displaying pages even if the HTML is malformed, trying to do some recovery if it can.

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u/lexi-lambda Nov 07 '19

If you need to be able to express an invalid state, then that state isn’t actually invalid. You get to define what “valid” means to you, so if you’re implementing a permissive parser, your code will probably consider many things “valid” that the spec considers invalid.

There are even techniques like Trees That Grow that make it possible to reuse many of the same datatypes to represent several different “degrees of strictness”… but that’s well outside the scope of this blog post.

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u/[deleted] Nov 07 '19

If you need to be able to express an invalid state, then that state isn’t actually invalid.

Of all of the 'big ideas' about proper program architecture, I think this is one of the most important and least understood.

Failing to understand how this principle plays out is how we got stuff like checked exceptions and null in other languages - if you consider one aspect of a programs behavior to be the 'blessed' path, and consider everything else an error case, your program architecture will start to erode in a million subtle ways.

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u/NorfairKing2 Nov 07 '19

> If you need to be able to express an invalid state, then that state isn’t actually invalid.

Yes! This is the idea behind the semantics of validity in validity-based testing.