On Type Inference


This is a comment I recently saw on a headline-grabbing article about Swift: –

I also don’t think that “type inferring” is of great use. If you cannot be bothered to hack in a variable’s data type, maybe you should not develop software in the first place.

I was so infuriated by this comment that I ended up writing this blog post. The thing that infuriated me so much was more the arrogance of the second sentence as much as the ignorance of why having type inference can be such a massive productivity gain.

Here are three versions of the same sentence written in three different ways – slightly contrived as usual to make my point…

  • Hello, I’m Java. I have a bag of apples. In this bag of apples there are five apples.
  • Hello, I’m C# 3. I have a big of apples that has five apples in it.
  • Hello, I’m F#. I have a bag of five apples.

These statements don’t actually translate directly to code but you get the idea. Why would you need to write out the above sentence in either of the latter ways? Are you learning the language from a school textbook? Or using it in a professional & fluent manner?

C# can declare locally-scope implict variables and create implicit arrays. F# can do full H-M type inference. Java can’t do much of anything really. Here’s the above sentences sort of written as code (the final example uses the same C#-style syntax to keep things consistent): –

The latter example is more in the F# vein, without braces, or keywords whose usage can be inferred.

The real power of type inference comes from the ability to chain multiple functions together in a succinct manner, and then at some point in the future changing the type of one of those functions and not having to change any type signatures of dependent functions. It’s quite addictive, and once you get used to it it’s quite difficult to go back to an explicitly-typed language.

Last minute update: It turns out that the individual who I quoted above at the start of this post had confused type inference with dynamic typing. I suspect that this won’t the first or last person who has done this.

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