feat!: Use Grain exceptions instead of JS exceptions #565

ospencer · 2021-03-16T02:22:56Z

feat: Add ability to throw exceptions
feat: Add ability to register custom exception printers
feat: Add Exception stdlib with Exception.registerPrinter
fix: Use Is instead of Eq for match variant comparison
fix: export * with exceptions
chore: Only emit MatchFailure exception for partial matches

Closes #173, closes #301, closes #302.

Overview

This PR adds the ability to throw exceptions in Grain. (Previously, you could create exceptions, pattern match on them, use them with Result, etc., but you could not throw them.)

throw InvalidArgument("Your argument is no good!")

The toString value of the exception is printed when an error is thrown, but thanks to a new standard library included in this PR, the message can be overridden:

import Exception from "exception"

exception ErrorCode(Number)

Exception.registerPrinter(e => match (e) {
  ErrorCode(1) => Some("Custom message 1"),
  ErrorCode(2) => Some("Custom message 2"),
  ErrorCode(_) => Some("Unknown error code!"),
  _ => None // Allow other printers to handle exceptions we don't know about!
})

Details

Exceptions are managed by stdlib/runtime/exception (not to be confused with stdlib/exception). This module maintains a pointer to the list of available error printers, defines some exceptions to be used by the runtime, and defines a printer for those exceptions. As toString is not available yet, it exports a method that allows Pervasives to register the "base" exception printer. If Pervasives is not used (such as via the --no-pervasives flag), exceptions are printed as the generic "GrainException".

If you want to know why the base set of exceptions are defined in that module and not built directly into the compiler, we can have a virtual drink and I'll tell you the tale of how I went down that rabbit hole.

There were a handful of bugs that I needed to fix to get this together, but I'll point them out with some comments.

If this is approved by the core team, there's a few issues that I'll want to open:

There's currently no way to re-export an exception. We've talked in the past about reworking how values vs types are imported/exported, and I believe we'll need to revisit that conversation. It's not a huge deal right now, as runtime functions can just import the exception and throw it. It will, however, be problematic in the future when WebAssembly exception handling lands and users will want to catch those errors—they should not be importing anything from stdlib/runtime.
MatchFailure wasn't actually being thrown on a match failure—we have a custom node in the match compiler that handles that case. That was a bigger refactor than what I thought should go in this PR, so I left it alone for now.
We should refactor most stdlib code to throw real errors instead of just failing.

ospencer · 2021-03-16T02:25:37Z

compiler/src/codegen/compcore.re

@@ -1969,7 +1973,7 @@ let allocate_array_n = (wasm_mod, env, num_elts, elt) => {
          compiled_num_elts(),
          Expression.const(wasm_mod, encoded_const_int32(0)),
        ),
-        InvalidArgument,
+        IndexOutOfBounds,


For these two, it didn't seem worth it to me to set up calling an exception with arguments in the compiler, when this is just going to be rewritten into Grain soon anyway. I feel that IndexOutOfBounds is also an appropriate error here until this is moved over. If anyone really cares, I can do it sooner.

ospencer · 2021-03-16T02:27:32Z

compiler/src/codegen/transl_anf.re

+        switch (partial) {
+        | Partial => [
+            {
+              instr_desc: MError(Runtime_errors.MatchFailure, [compiled_arg]),


Like I mentioned in the description, this isn't actually the error that's happening on a match failure. I did have to do this (sensible) change though to allow the exception handler to use match and not depend on itself.

ospencer · 2021-03-16T02:29:42Z

compiler/src/middle_end/matchcomp.re

@@ -375,13 +375,7 @@ module MatchTreeCompiler = {
      );
    | Fail =>
      /* FIXME: We need a "throw error" node in ANF */


Even though I made this trap instead of moving on silently, I left this note here because that's what needs to happen to throw MatchFailure instead of just unreachable.

ospencer · 2021-03-16T02:33:44Z

compiler/src/middle_end/matchcomp.re

@@ -502,7 +496,7 @@ module MatchTreeCompiler = {
                cmp_id_name,
                Comp.prim2(
                  ~allocation_type=StackAllocated(WasmI32),
-                  Eq,
+                  Is,


Calling Pervasives.(==) was unnecessary here (and prevented pattern matching on variants to happen in runtime mode). This actually will speed up pattern matching by a non-trivial amount.

ospencer · 2021-03-16T02:34:32Z

compiler/test/test_end_to_end.re

@@ -2182,5 +2204,4 @@ let tests =
  @ export_tests
  @ comment_tests
  @ number_tests
-  @ exception_tests
-  @ export_tests;


export_tests was in the list twice! Sneaky sneaky.

ospencer · 2021-03-16T02:35:05Z

runtime/src/core/closures.js

@@ -1,7 +1,5 @@
 import { managedMemory, grainModule } from "../runtime";


This file isn't used, but I kept it in because it can be useful for FFI.

ospencer · 2021-03-16T02:36:09Z

stdlib/runtime/exception.gr

+
+export let mut printers = 0n
+
+export let registerBasePrinter = (f) => {


Arguably this should be dangerouslyRegisterBasePrinter.

ospencer · 2021-03-16T02:36:49Z

stdlib/runtime/wasi.gr

+
+exception SystemError(Number)
+
+let stringOfSystemError = (code) => {


This is honestly one the biggest things that I'm excited for.

phated

This PR is so spicy! 🌶️ Love it!

phated · 2021-03-23T23:08:28Z

compiler/src/parsing/lexer.mll

@@ -177,7 +177,7 @@ rule token = parse
  | "fail" { FAIL }
  | "exception" { EXCEPTION }
  | "try" { TRY }
-  | "raise" { RAISE }
+  | "throw" { THROW }


Does this lexer change make this a breaking? (I forget if this PR includes a breaking note)

Mmm, yeah it does (if you used throw as an identifier).

compiler/src/typed/builtin_types.re

runtime/src/core/closures.js

stdlib/runtime/exception.gr

stdlib/string.gr

stdlib/runtime/gc.gr

phated · 2021-03-23T23:45:01Z

stdlib/char.gr


 export *

+exception MalformedUtf8


Should this have a printer?

I thought the default printer was fine, but I suppose I could see a printer for it.

stdlib/runtime/exception.gr

feat: Add ability to `throw` exceptions feat: Add ability to register custom exception printers feat: Add Exception stdlib with Exception.registerPrinter fix: Use Is instead of Eq for match variant comparison fix: `export *` with exceptions chore: Only emit MatchFailure exception for partial matches

phated

Looks like the tests are passing now, so take this ⚔️ (it's dangerous to go alone)!

ospencer requested a review from a team March 16, 2021 02:22

ospencer self-assigned this Mar 16, 2021

ospencer commented Mar 16, 2021

View reviewed changes

ospencer force-pushed the refactor-runtime-setup branch 2 times, most recently from 77ecea7 to 8ae6706 Compare March 23, 2021 03:00

ospencer force-pushed the grain-exception branch from 65c6e16 to 17b936b Compare March 23, 2021 03:21

Base automatically changed from refactor-runtime-setup to main March 23, 2021 22:48

ospencer force-pushed the grain-exception branch from 17b936b to e78e811 Compare March 23, 2021 22:49

phated requested changes Mar 23, 2021

View reviewed changes

ospencer force-pushed the grain-exception branch from b574318 to cf04724 Compare March 24, 2021 04:52

phated changed the title ~~feat: Use Grain exceptions instead of JS exceptions~~ feat!: Use Grain exceptions instead of JS exceptions Mar 24, 2021

ospencer force-pushed the grain-exception branch from cf04724 to e646fb0 Compare March 24, 2021 04:59

phated approved these changes Mar 24, 2021

View reviewed changes

ospencer merged commit 1f1cd4a into main Mar 24, 2021

ospencer deleted the grain-exception branch March 24, 2021 05:13

phated mentioned this pull request Apr 19, 2021

chore: release testing-release #595

Closed

github-actions bot mentioned this pull request Apr 20, 2021

chore: release main #607

Merged

github-actions bot mentioned this pull request May 31, 2022

chore: release main #1292

Merged

github-actions bot mentioned this pull request Jul 25, 2022

chore: release main spotandjake/grain#1

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feat!: Use Grain exceptions instead of JS exceptions #565

feat!: Use Grain exceptions instead of JS exceptions #565

ospencer commented Mar 16, 2021 •

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		@@ -1,7 +1,5 @@
		import { managedMemory, grainModule } from "../runtime";


		export let mut printers = 0n

		export let registerBasePrinter = (f) => {


		exception SystemError(Number)

		let stringOfSystemError = (code) => {

feat!: Use Grain exceptions instead of JS exceptions #565

feat!: Use Grain exceptions instead of JS exceptions #565

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ospencer commented Mar 16, 2021 • edited Loading

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ospencer commented Mar 16, 2021 •

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