Groups   208 of 99+      julia-users › scope using let and local global variables 9 posts by 3 authors      Michael Mayo    12 10 14   Hi folks,  I have the following code fragment:  x 1 let x 2   println x               Line 1   exp : x+1    println eval exp        Line 2 end  It contains two variables both named x, one inside the scope defined by let, and one at global scope.  If I run this code the output is: 2 2  This indicates that  i  that line 1 is using the local version of x, and  ii  that line 2 is using the global version of x.  If I remove this global x I now get an error because eval   is looking for the global x which no longer exists:  let x 2   println x               Line 1   exp : x+1    println eval exp        Line 2 end  2 ERROR: x not defined  My question: when evaluating an expression using eval   such as line 2, how can I force Julia to use the local  not global  version of x and thus avoid this error?  Thanks Mike     Jameson    12 11 14   eval, by design, doesn't work that way. there are just too many better alternatives. typically, an anonymous function   lambda is the best and most direct replacement:  let x 2   println x               Line 1   exp      -  x+1   println exp         Line 2 end       Michael Mayo    12 11 14   Thanks, but its not quite what I'm looking for. I want to be able to edit the Expr tree and then evaluate different expressions using variables defined in the local scope,not the global scope  e.g. for genetic programming, where random changes to an expression are repeatedly evaluated to find the best one . Using anonymous functions could work but modifying the .code property of an anonymous function looks much more complex than modifying the Expr types.  Anyway thanks for your answer, maybe your suggestion is the only possible way to achieve this!  Mike       Jameson    12 11 14   I'm not quite sure what a genetic program of that sort would look like. I would be interested to hear if you get something out of it.  Another alternative is to use a module as the environment:  module MyEnv end eval MyEnv, : code block    This is  roughly  how the REPL is implemented to work.      Mike Innes    12 11 14   You can do this just fine, but you have to be explicit about what variables you want to pass in, e.g.  let x 2   exp : x+1    eval : let x   $x; $exp; end   end  If you want to call the expression with multiple inputs, wrap it in a function:  let x 2   exp : x+1    f   eval : x -  $exp     f x  end       Michael Mayo    12 11 14   Thanks for both answers! I figured out a slightly different way of doing it by putting the let assignments into a string with a  nothing  expression, parsing the string, and then inserting the actual expression to be evaluated into the correct place in the let block:  function run assignments,program    program_string  let    for pair in assignments     program_string  $ program_string  $ pair 1   $ pair 2  ;    end   program_string  $ program_string  nothing; end    program_final parse program_string    program_final.args 1 .args end  program   eval program_final  end  I can now evaluate the same expression with different inputs in parallel without worrying that they might conflict because all the variables are local, e.g.:  pmap dict- run dict,: x+y y  ,   :x  2,:y  5 , :x  6,:y  10    2-element Array Any,1 :   27  106  Thanks for your help! Mike      Mike Innes    12 11 14   Great that you got this working, but I strongly recommend working with expression objects here as opposed to strings. It's likely to be more robust and will mean you can use data that isn't parseable  i.e. most things other than numbers  as inputs.      Michael Mayo    12 11 14   Yes you are right! The final version with the strings removed:  function run assignments,program    program_final Expr :block    args Expr     for pair in assignments     append! args,  Expr :   , pair 1 , pair 2       end   append! args, program     program_final.args args   eval program_final  end  Turns out that making a let expression is not required; a simple block expression does the job. Mike      Michael Mayo    12 11 14   My apologies, a let block  is  required otherwise the variables are being defined at global scope. The corrected function:  function run assignments,program    block Expr :block    args Expr     for pair in assignments     append! args,  Expr :   , pair 1 , pair 2       end   append! args, program     block.args args   program_final Expr :let, block    eval program_final  end  Mike