{"action":"created","comment":{"url":"https://api.github.com/repos/JuliaLang/julia/pulls/comments/29189543","id":29189543,"diff_hunk":"@@ -157,6 +157,7 @@ typedef struct {\n     unsigned short how:2;\n     unsigned short isshared:1;  // data is shared by multiple Arrays\n     unsigned short isaligned:1; // data allocated with memalign\n+    unsigned short fixed_len:1; // data cannot be resized (1d only)","path":"src/julia.h","position":13,"original_position":13,"commit_id":"1f6e98796f43dfd47df62977800c08a4407c61ac","original_commit_id":"1f6e98796f43dfd47df62977800c08a4407c61ac","user":{"login":"JeffBezanson","id":744556,"avatar_url":"https://avatars.githubusercontent.com/u/744556?v=3","gravatar_id":"","url":"https://api.github.com/users/JeffBezanson","html_url":"https://github.com/JeffBezanson","followers_url":"https://api.github.com/users/JeffBezanson/followers","following_url":"https://api.github.com/users/JeffBezanson/following{/other_user}","gists_url":"https://api.github.com/users/JeffBezanson/gists{/gist_id}","starred_url":"https://api.github.com/users/JeffBezanson/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/JeffBezanson/subscriptions","organizations_url":"https://api.github.com/users/JeffBezanson/orgs","repos_url":"https://api.github.com/users/JeffBezanson/repos","events_url":"https://api.github.com/users/JeffBezanson/events{/privacy}","received_events_url":"https://api.github.com/users/JeffBezanson/received_events","type":"User","site_admin":false},"body":"Could we set `isshared` instead, which I think also prevents growing?","created_at":"2015-04-27T20:50:33Z","updated_at":"2015-04-27T20:50:33Z","html_url":"https://github.com/JuliaLang/julia/pull/11034#discussion_r29189543","pull_request_url":"https://api.github.com/repos/JuliaLang/julia/pulls/11034","_links":{"self":{"href":"https://api.github.com/repos/JuliaLang/julia/pulls/comments/29189543"},"html":{"href":"https://github.com/JuliaLang/julia/pull/11034#discussion_r29189543"},"pull_request":{"href":"https://api.github.com/repos/JuliaLang/julia/pulls/11034"}}},"pull_request":{"url":"https://api.github.com/repos/JuliaLang/julia/pulls/11034","id":34204781,"html_url":"https://github.com/JuliaLang/julia/pull/11034","diff_url":"https://github.com/JuliaLang/julia/pull/11034.diff","patch_url":"https://github.com/JuliaLang/julia/pull/11034.patch","issue_url":"https://api.github.com/repos/JuliaLang/julia/issues/11034","number":11034,"state":"open","locked":false,"title":"Coherent map and comprehension, the imminent death of type_goto","user":{"login":"carnaval","id":5307791,"avatar_url":"https://avatars.githubusercontent.com/u/5307791?v=3","gravatar_id":"","url":"https://api.github.com/users/carnaval","html_url":"https://github.com/carnaval","followers_url":"https://api.github.com/users/carnaval/followers","following_url":"https://api.github.com/users/carnaval/following{/other_user}","gists_url":"https://api.github.com/users/carnaval/gists{/gist_id}","starred_url":"https://api.github.com/users/carnaval/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/carnaval/subscriptions","organizations_url":"https://api.github.com/users/carnaval/orgs","repos_url":"https://api.github.com/users/carnaval/repos","events_url":"https://api.github.com/users/carnaval/events{/privacy}","received_events_url":"https://api.github.com/users/carnaval/received_events","type":"User","site_admin":false},"body":"Let's have this be the basis for discussion. The properties I feel we need are :\r\n(1) `map(Y->X,Z)` and `[X for Y in Z]` have the same semantics\r\n(2) We can specify julia's runtime execution without mentions of type inference\r\n(3) The case where `isempty(Z)` should be the least surprising possible for the user, both from a semantic and performance standpoint\r\n\r\nChoosing a type for `map(f,T[])` gives us then 3 choices per rule (2) : either Any, Bottom, or T. Choosing T feels completly arbitrary to me, and, as I explained in https://github.com/JuliaLang/julia/issues/7258#issuecomment-96288936, Bottom is quite convenient.\r\nChoosing Any is a performance pitfall which violates rule (3), unless we always return Vector{Any} for not-explicitly-typed `map`s and comprehensions.\r\n\r\nChoosing Bottom has a problem with (3) which is that the empty case is no longer resizable which is \"fixed\" by making all the cases immutable length : this is what this PR implements.\r\n\r\nI then see essentially only two options :\r\nA) this PR\r\nB) the simpler rule of returning only Any arrays except when explicitly asked for a return type (@jakebolewski's prefered solution).\r\n\r\nAny opinion on this topic would have to argue one of the following : A) is the way to go, B) is the way to go, one of (1-3) should be broken, my reasoning is 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