Monkey Tips - Default Types And Implicit Type Conversion

This one is mostly for people new to Monkey and coming from other languages. Here's some Monkey code:

Local myVarA = 1
Local myVarB = 1.5
Local myVarC = True

If you're like me when I first saw Monkey, I assumed that what I was seeing there was type inference. If I don't declare the variable type then you'll work it out for me? Awesome. So why does this not work?

Local myVarD = "Hello World"

The answer is that no type inference is occurring. If you don't declare a variable's type then Monkey will default that type to be an integer. Further, Monkey performs implicit type conversion between some primitives. Floats can be converted to Ints and vice versa and Bools can be converted to Ints  (but not Ints to Bools). You don't even get a warning that this is happening and the first three examples above all compile happily with you potentially none the wiser that you've just declared three Ints with a value of 1.

Now some people might consider that sort of thing convenient. For me, that the float and boolean to int conversions just go by without Monkey even raising an eyebrow is convenient like leaving your roller skates at the top of the stairs so you don't have to go rummaging in the wardrobe. You absolutely will end up accidentally lopping off your float precision at some point.

There is a partial answer to this danger and that's to switch on "strict mode" by adding the Strict keyword to the top of your file. This mode will raise an error if you don't declare a type - although it has to be said that it's not a very good error: "Illegal type expression" rather than something a little more descriptive like "Type is undefined". Strict mode doesn't do anything about implicit type conversion though and it still happens silently in literal assignments or calculations.

I should mention that Monkey does offer type inference, but you have to explicitly invoke it via the ":=" operator:

Local thisIsAFloat := 1.5

<sarc>Which, I'm sure you'll agree, is something you'd never forget or mess up. Even if you did miss the colon, you couldn't fail to spot such a glaring error, right?</sarc> Seriously, just use Strict.