What does this clojure.core.typed type error mean?

clojure, clojure-core.typed, types

Solution

The first part of the error tells you this concerns a call to `clojure.core/keys`. You can lookup the type with `(cf keys)`.

` (All [k] [(Map k Any) -> (Seq k) :object {:id 0 :path [Keys]}]) `

The error is basically summarising this polymorphic type juxtaposed with the actual types provided to the function.

The `Polymorphic Variables` list all the variables in the `All` binder and their type bounds. `k` is the only variable, which has essentially no bounds, so `k` is displayed.

The `Domains` list all the parameter types (to the left of the `->`) in order. If multiple arities were specified with `Fn`, each parameter list would be displayed sequentially.

The `Arguments` show you what types are actually passed to the function. The code at 14:23 is presumably this call `(keys hm)`, so the type of the first argument is displayed: `(t/Option (t/Map k v1))`

The `Ranges` list all the return types (to the right of the `->`) in order.

Sometimes there is an `Expected` type, which must match up with a `Range` in the same way all the `Arguments` must match up with a `Domain`.

We can diagnose this error by comparing the `Domains` with the `Arguments`. The list of `Arguments` must fit under one list of `Domains`, and the match is attempted top-down. If no `Domains` fit with the `Arguments`, we get a type error like this; it's often too complicated to pinpoint exactly where the constraint algorithm failed, so the user is presented with a lot of information.

In this case, we try and fit the argument `(t/Option (t/Map k v1))` under the domain `(t/Map k Any)`, which fails. This is because the argument `(t/Option (t/Map k v1))` is the same as `(U nil (t/Map k v1))`, and `nil` does not fit under the domain `(t/Map k Any)`.

There are a couple of ways to fix this. You basically need to ensure `keys` isn't passed `nil`. Here's one option:

` (keys (or hm {})) `

Problem

Original source