Typed Clojure 1.0.17 introduces custom type error messages for macros.
For example, the following code yields a type error--because you can't destructure a set:
(let [[a] #{:foo}]
a)Previously, Typed Clojure would expand this code and type check its expansion. The resulting error message is too low-level and quite difficult to decipher, especially in you are unfamiliar with destructure's implementation:
$ lein with-profile +old-typed typed check
...
Type Error (file:/Users/ambrose/Projects/scratch/let-error-demo/src/let_error_demo/core.clj:6:3)
Polymorphic static method clojure.lang.RT/nth could not be applied to arguments:
Polymorphic Variables:
x
y
Domains:
(t/U (t/I Sequential (Seqable x) (t/CountRange 1)) (t/I (Indexed x) (t/CountRange 1))) (t/Val 0) t/Any
(t/U (Indexed x) nil (t/I Sequential (Seqable x))) t/Int y
Arguments:
(t/HSet #{:foo}) (t/Val 0) nil
Ranges:
x
(t/U x y)
in:
(clojure.lang.RT/nth vec__30544 0 nil)
...The new approach--introduced in 1.0.17--uses a custom typing rule for let to inject surface syntax into the error message:
$ lein typed check
...
Type Error (file:/Users/ambrose/Projects/scratch/let-error-demo/src/let_error_demo/core.clj:6:3)
The type `(t/HSet #{:foo})` cannot be destructured via syntax `[a]` because the type cannot be passed as the first argument of `nth`.`
in:
(let [[a] #{:foo}] a)
...Notice the entire let form is in the message, as well as the involved type and destructuring syntax.
Copyright © 2021 Ambrose Bonnaire-Sergeant
This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at http://www.eclipse.org/legal/epl-2.0.
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