I learned a lot on this subject from a couple of Stefan's posts in a thread that occurred a while back:
https://groups.google.com/d/msg/julia-users/bldp27KJCjY/0T5kM21wSHsJ https://groups.google.com/d/msg/julia-users/bldp27KJCjY/LHM_MKaXw2YJ On Wednesday, November 19, 2014 11:09:56 AM UTC-8, Frederick wrote: > > Will do. Thanks, John > > On Wednesday, November 19, 2014 11:45:17 AM UTC-6, John Myles White wrote: >> >> Hi Frederick, >> >> You'll want to read a bit more about what invariance means in computer >> science: >> http://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science) >> >> In particular, Array{S} <: Array{T} is false for all types S and T with S >> != T in Julia. This isn't an inconsistency -- it's an important part of >> what makes Julia Julia. >> >> -- John >> >> On Nov 19, 2014, at 5:35 PM, Frederick <fpmen...@hotmail.com> wrote: >> >> >> >> Hello, >> >> While I understand that Strings will be overhauled in 0.4 >> <https://groups.google.com/forum/?fromgroups=#!searchin/julia-users/substrings/julia-users/xCB8LeGFYcw/lNpEi3wh2qYJ>, >> >> I would like help in understanding why the following: >> >> First there is this inconsistency: >> >> >> In [16]: SubString{ASCIIString} <: String >> >> Out[16]: true >> In [18]: Array{SubString{ASCIIString},1}<:Array{String,1} >> >> Out[16]: false >> >> Leading to the following, which didn't work... >> In [19]: >> >> function firstletter(stringarray::Array{String,1}) >> [s[1] for s in stringarray] >> end >> >> stringarray = split("ABCDE FGHIJ KLMNO PQRST"," ") >> print(typeof(stringarray)) >> firstletter(stringarray) >> >> Array{SubString{ASCIIString},1} >> >> `firstletter` has no method matching firstletter(::Array{SubString{ >> ASCIIString},1}) >> while loading In[19], in expression starting on line 7 >> >> >> There is no clear documentation on Substrings, convert operations, nor >> what types are subtypes of what other types. (Although I found code to >> generate type graph <http://pastebin.com/En28tWjS>, I did not see >> SubStrings in the graph.) >> >> From what I can tell from the postings in this group and others, the >> above should work according to Leah in String from a substring Jul 21 >> (2014?) >> <https://groups.google.com/forum/?fromgroups=#!searchin/julia-users/array$20substring/julia-users/AlIbGTS_15w/0l5Qoh2bjfoJ> >> >> and Patrick O'Leary | 4 Feb 06:35 2014 in SubString{ASCIIString} causing >> issues <http://comments.gmane.org/gmane.comp.lang.julia.devel/13400>. >> But it seems something about Arrays causes the difficulty. Jack Holland >> encountered a similar problem in Type variables acting differently when >> accessed through an array Jun 17 (2014?) >> <https://groups.google.com/forum/?fromgroups=#!searchin/julia-users/array$20substring/julia-users/VvVRW9Llc_0/JqcmlmhQgyYJ> >> . >> >> Thanks for the help, Frederick >> >> >>
