spurious error when using the result of string.gsub

[fperrad]

$ cat gsub.tl 
global function translate (str: any)
     if type(str) == 'string' then
         -- type not narrowed, so there are error in 2 following lines
         str = str:gsub('a', 'A')
         str = str:gsub('b', 'B')
     end
     if str is string then
         -- type narrowed, so no error at the following line
         str = str:gsub('c', 'C')
         -- now spurious error at the following line, look likes the return of `gsub` is not a string
         str = str:gsub('d', 'D')
     end
     if type(str) == 'string' then
         -- type not narrowed, so there are error in 2 following lines
         str = string.gsub(str, 'e', 'E')
         str = string.gsub(str, 'f', 'F')
     end
     if str is string then
         -- type narrowed, so no error at the following line
         str = string.gsub(str, 'g', 'G')
         -- now spurious error at the following line, look likes the return of `gsub` is not a string
         str = string.gsub(str, 'h', 'H')
     end
end
$ tl gen --check gsub.tl 
Wrote: gsub.lua
========================================
6 errors:
gsub.tl:4:20: cannot index key 'gsub' in any 'str' of type <any type>
gsub.tl:5:20: cannot index key 'gsub' in any 'str' of type <any type>
gsub.tl:11:20: cannot index key 'gsub' in any 'str' of type <any type>
gsub.tl:15:28: argument 1: got <any type>, expected string
gsub.tl:16:28: argument 1: got <any type>, expected string
gsub.tl:22:28: argument 1: got <any type>, expected string

[hishamhm]

• type() does not narrow, that's expected (because it could be used it complex expressions with side-effects), that's why I added the more restricted is operator, which reliably has the shape VAR is TYPE
• The compiler does not do generalized implicit flow typing: it does not update the known type of a variable on each assignment (it's a statically-typed language, after all, not a type checker for a dynamically-typed one). There are a few situations where you get "explicit" flow typing, such as when using if VAR is TYPE: that narrows str from any to string. By assigning to str, however, you drop the narrowing and revert the variable back to the declared type, which is any. Performing generalized flow typing (that works across conditions, loops, function calls with side-effects, etc) would require a much more complex dataflow analysis than Teal's current single-pass compiler does.
• I have played with extending flow typing to assignment results, but there were significant edge cases — for instance, I could make it work for the examples above, but it would then blow up in a slightly different example (especially as you add subtyping relationships).
• The any type is not a very useful or convenient one (any accepts anything, but other types don't accept any), and isn't really intended to give you the convenience of dynamically-typed programming. Your best bet for now is to check it with is (or hard-cast with as if you're sure of the type) and then assign to a local with the known type.

[fperrad]

many thanks for these clarifications.