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bugfix: bound subtyping depth to avoid reliance on unpredictable stac…
…k overflow (#4798) * repro for subtyping loop due to polymorhic recursion * Update test/fail/polyrec.mo * Update test/fail/polyrec.mo Co-authored-by: Gabor Greif <[email protected]> * hack subtyping to accept polyrec.mo by treating equivalent instanstations of the same constructor as related by subtyping * add bound to subtyping instead of relying on unpredictable and expensive OOM/stackoverflow * tidy * missin file? * turn overflow into error (WIP) * add error code desc; use dedicated exception Undecided * Update src/lang_utils/error_codes/M0200.md * check issue-3057 * Update test/fail/issue-3057.mo --------- Co-authored-by: Gabor Greif <[email protected]>
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,44 @@ | ||
| # M0200 | ||
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| This error means that the compiler encountered a subtyping, equivalence or constructor equivalence problem that it cannot decide is true or false in a timely manner. | ||
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| This is due to a limitation of the type system and may require you to rewrite | ||
| some of your code to avoid the problem. | ||
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| The most like cause is a recursive type or class whose definition involves | ||
| instantiating the type or class with nested type parameters. | ||
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| For example, this definition of `Box<T>` | ||
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| ``` motoko | ||
| class Box<T>(v : T) { | ||
| public let value = t; | ||
| public func map<R>(f : T -> R) : Box<R> { | ||
| Box<R>(f t) | ||
| }; | ||
| } | ||
| ``` | ||
| is problematic because `Box<R>` is instantiated at `R`, an inner type parameter, while | ||
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| ``` motoko | ||
| class Box<T>(v : T) { | ||
| public let value = v; | ||
| public func map(f : T -> T) : Box<T> { | ||
| Box<T>(f value) | ||
| }; | ||
| } | ||
| ``` | ||
| is accepted (but also less useful). | ||
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| Another workaround is to define the problematic method as a separate | ||
| function, outside of the class: | ||
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| ``` motoko | ||
| class Box<T>(v : T) { | ||
| public let value = v | ||
| }; | ||
| func map<T, R>(b : Box<T>, f : T -> R) : Box<R> { | ||
| Box<R>(f(b.value)) | ||
| }; | ||
| ``` |
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