[pbiggar]

I work on Dark (https://darklang.com), which uses a similar concept about holes to Hazel. I've looked at Hazel but amn't totally familiar, so take this answer in that context.

In the first case, the hole has a type. I believe this type is inferred by the compiler/editor/type-checker, and I believe it will tell you if they type is wrong (in Dark, we'll tell you if the type is wrong, though we'll still let you put that variable in that hole to support the transition from one valid state to another).

In the configuration file example, you can't change the type: these are statically typed functional languages, and they don't permit "later-time" type changes.