Stream: bugs

Topic: Compiler crash from branching recursive nominal types

Austin Davis (Jul 24 2025 at 20:12):

The following code works fine:

TypeA := [
    X TypeB,
    Y TypeB,
    Nil,
]

TypeB := TypeA

But this code fails:

TypeA := [
    X TypeA,
    Y TypeB,
    Nil,
]

TypeB := TypeA

The only difference between them is that, in the second block, the X and Y tags contain _different_ inner types (TypeA and TypeB), and in the first block, the X and Y tags contain _the same_ inner type (TypeA).

In the first case, the compiler accepts the type with no errors. In the second case, we get the following error message:

thread '<unknown>' has overflowed its stack
fatal runtime error: stack overflow
zsh: abort      roc check

Expectation:

Both kinds of recursive data structures should be accepted by the compiler. The issue does not seem to be with mutual recursion in general, but rather this special case of multiple distinct inner recursive types.

Result:

Compiler crashes due to stack overflow when running roc check.

Notes:

Compiler release:
roc_nightly-macos_apple_silicon-2025-03-22-c47a8e9cdac

Austin Davis (Jul 25 2025 at 02:26):

I should also add that this bug basically makes it impossible to represent any kind of complex recursive tree structure in Roc, including the AST in the compiler project I’m working on. I don’t think there is a real workaround at this point.

Austin Davis (Jul 25 2025 at 02:38):

I suppose it may be possible to emulate GADTs with phantom types in order to keep the entire recursive structure within a single unified type declaration. I might try that just to see if it’s possible. But that is extremely far from ergonomic, unfortunately.

Anton (Jul 25 2025 at 10:38):

Hi @Austin Davis,
This will be fixed in the new compiler: #beginners > Help implementing recursive type? @ 💬

Austin Davis (Jul 25 2025 at 14:16):

Awesome, can’t wait for the new compiler release!!

Last updated: Jul 26 2025 at 12:14 UTC