type checking experiments (parser samples)

[Asko Kauppi <askok@...>]

There was some discussion about type checking in the LuaOhSix  
workshop.  I've now experimented with the concept (at a patch level,  
on 5.1.1) and come to the point where it's better to stop for a  
while, and tell what I've been up to.

Here are syntactic samples, they do parse but don't get runtime- 
checked, yet (there's a couple greater hurdles on that..)

  -- -1<=v<=3 on input, and -2<= <=2 on return
  --
  local function mynut( v :range(-1,3) )
    : range(-2,2)
    return v-1
  end

  -- more parameters, multiple return types
  --
  local function mydear( t :string_or_table, i :int, j :int_opt,  
n :number_opt )
    : string : table : int_opt

    --return      -- fail
    --return 1,2,3    --fail
    --return "",{i=5}     -- ok
    return "",{i=5},33  -- ok
end

  -- table structure constraint (can also be covered by a custom  
constraint function)
  --
  local myfear= function( t :{ a=int, b=string }, v :int_opt )
      ..
  end

  local function myfear( t :myab, v :int_opt )
      ..
  end

I first planned only to have the function entry/return values checked  
(and this can be done in pure Lua, it's not much trouble..) but later  
extended to concept to what if _any_ Lua (local) variable could be  
tagged as of certain type(s), and that tested each time the variable  
is given a new value.

  local myvar :int, mystr :string= 10, 0      -- will become "0"

  assert.number( myvar )
  assert.string( mystr )  -- mystr should have implicitly gotten 0- 
>"0" conversion

One could use such technique only where wanting, of course. The  
purpose would be to bring Lua a bit towards the static typed  
direction, but carefully.

There are some neat side-effects to this. :)  Such as ability to  
implicitly make those '(%d+)' captures become numbers, without  
harming the rest of the Lua population. That was not why I started to  
look into this, though.

  local a:number, b:any= string.match( "123 456", "^(%d+)%s+(%d+)" )

Will finish this "sometime later".  Currently, it's here just to let  
you know, and in case someone's up to a similar study.

- asko

Attachment: sample.lua
Description: Binary data