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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Error object returned by pcall() declared to always be a string","articleBody":"#### Description\r\n\r\n`pcall()` is [declared](https://github.com/TypeScriptToLua/lua-types/blob/master/core/global.d.ts#L180) to return `LuaMultiReturn\u003c[true, R] | [false, string]\u003e`, but in Lua, the error object can be anything. This leads to problem when the error object is used in a context where it matters whether it can be something other than a string.\r\n\r\n#### To reproduce\r\n\r\nConsider the following example ([playground](https://typescripttolua.github.io/play/#code/GYVwdgxgLglg9mABMOcAUUAWMDOAuRANzhgBMBKRAbwChF7EsAnOAd2oF8auaAbAUyiIm-HCF5CAvIgAOEAIa9eaFHHI0aMYIjQBCEWIkBtAAwBdSrQaymMMFDQAifkxZMCjxAGpho8VCMARgtuGiA)):\r\n\r\n```typescript\r\nfunction foo(this: void) {\r\n    throw {}\r\n}\r\n\r\nlet result = pcall(foo)\r\n\r\nif (!result[0]) {\r\n    print(\"error: \" + result[1])\r\n}\r\n```\r\n\r\nFunction `foo()` throws an empty object. To allow concatenation with a string, a call to `tostring()` needs to be inserted, but it isn't. So the concatenation in the argument to `print()` fails:\r\n\r\n```\r\nLua execution error:\r\n\t[string \"--[[ Generated with https://github.com/TypeSc...\"]:7: attempt to concatenate a table value (field '?')\r\n```\r\n\r\nIf the correct type is added to the declaration of `result`, the call to `tostring()` is inserted:\r\n\r\n```typescript\r\nlet result: [true, void] | [false, unknown] = pcall(foo)\r\n```\r\n\r\nGenerated code:\r\n\r\n```lua\r\n--[[ Generated with https://github.com/TypeScriptToLua/TypeScriptToLua ]]\r\nfunction foo()\r\n    error({}, 0)\r\nend\r\nresult = {pcall(foo)}\r\nif not result[1] then\r\n    print(\"error: \" .. tostring(result[2]))\r\nend\r\n```\r\n\r\n#### Expected behavior\r\n\r\nI think that the return type of `pcall()` should be changed to `LuaMultiReturn\u003c[true, R] | [false, unknown]\u003e`. The same applies to [`coroutine.resume()`](https://github.com/TypeScriptToLua/lua-types/blob/b224030d184a233c8b2eba021b04f07987b631be/core/coroutine.d.ts#L28).","author":{"url":"https://github.com/Feuermurmel","@type":"Person","name":"Feuermurmel"},"datePublished":"2024-11-06T13:17:29.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/79/lua-types/issues/79"}