[RFC 0135] Custom asserts

rfcs/0135-custom-asserts.md

@@ -0,0 +1,116 @@
+---
+feature: custom-asserts
+start-date: 2022-09-21
+author: Anselm Schüler
+co-authors: None
+shepherd-team: None
+shepherd-leader: None
+related-issues: None
+---
+
+# Summary
+[summary]: #summary
+
+Allow users to use attribute sets with a boolean attribute `success` and a string attribute `message` instead of a boolean in `assert …; …` expressions.
+
+# Motivation
+[motivation]: #motivation
+
+Since Nix is an untyped language, asserts are often needed to ensure that a function or program works correctly. However, the current assert system is unsuitable for more sophisticated error reporting that aims to inform the user as to what has happened.
+
+Consider a nixpkgs package expression that wants to validate its arguments. Currently, the best way to
+provide a custom error message is to use `assert … -> throw …; …`.
+This method has several disadvantages: Since an implication from a falsehood is always true, you are required
+to invert the condition. Additionally, since the assertion itself is not triggered by the error,
+the function of the `assert` keyword is reduced to providing an imperative shorthand for `seq`. This also means that by default,
+the error location is not printed, and there is no mention of an assert in the error message.
+Instead, expressions could use this more natural syntax:
+
+```nix
+{ foo, bar }:
+assert {
+  success = foo || bar;
+  message = "At least one of foo or bar must be set";
+};
+[ foo bar ]
+```
+
+Also consider, for instance, this simple type system:
+
+```nix
+rec {
+  assertType = type: locDescr: value:
+    if ! type.check value
+    then throw "Value at ${locDescr} was not of type ${type.name}"
+    else builtins.traceVerbose "Successfully checked type ${type.name} at ${locDescr}" value;
+  assertTypeSeq = type: locDescr: value1: value2:
+    builtins.seq (assertType type locDescr value1) value2;
+  intType = {
+    check = builtins.isInt;
+    name = "integer";
+  };
+  attrsOfType = subType: {
+    check = value:
+      builtins.isAttrs value
+      && builtins.all subType.check (builtins.attrValues value);
+    name = "attribute set of ${subType.name}";
+  };
+}
+```
+
+Users of this system would be forced to forgo the convenience of imperative-style `assert …; …` in favor of `seq`-like syntax in order to benefit from improved type errors. With this change, no longer! A variant `isType` function could be declared:
+
+```nix
+{
+  isType = type: locDescr: value:
+    let
+      success = type.check value;
+      message = if success
+        then "Value at ${locDescr} was not of type ${type.name}"
+        else "Successfully checked type ${type.name} at ${locDescr}";
+    in { inherit success message; };
+}
+```
+
+Compare three implementations of a function that only takes attribute sets of integers:
+
+```nix
+with types;
+{
+  onlyTakesAttrsOfInt1 = value:
+    assertType (attrsOfType intType) "onlyTakesAttrsOfInt1" value;
+  onlyTakesAttrsOfInt2 = value:
+    assertTypeSeq (attrsOfType intType) "onlyTakesAttrsOfInt1" value value;
+  onlyTakesAttrsOfInt3 = value:
+    assert isType (attrsOfType intType) "onlyTakesAttrsOfInt1" value;
+    value;
+}
+```
+
+While these implementations get more and more verbose, they also get more and more idiomatic and flexible.
+
+# Detailed design
+[design]: #detailed-design
+
+Allow users to use attribute sets with a boolean attribute `success` and a string attribute `message` instead of a boolean in `assert …; …` expressions.
+If the `success` attribute is false, the assertion fails with a message including the `message` attribute.
+
+This change requires no change to the language grammar.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- Implementation clutter
+- This could end up as a confusing and underutilized feature that hardly anybody knows about
+
+# Alternatives
+[alternatives]: #alternatives
+
+- Doing nothing and continuing to use whichever assertion method is most appropriate for a given use case
+- Deprecating `assert …; …` expressions in favour of user-made type systems
+
+# Unresolved questions
+[questions]: #unresolved-questions
+
+- What should the exact name of `message` and `success` be?
+- What should the error messsage be?