Create a ruby Proc from a string

Question

I want to define the block as a string, then create the lambda. The following example does not work. Is something like this possible?

code_string = "|x|x*2"

l = lambda {eval(code_string)}

l.call(3) => 6

I don't mean to sound rude, but why in the world do you want to do this? eval is almost never the best way to do something, for efficiency's sake if nothing else. — Chuck
– Chuck, Commented Mar 17, 2010 at 2:35
Sometimes you need to generate code "on-the-fly", often from a source that is not Ruby code. I use this technique to implement a transpiler. — Peter Camilleri
– Peter Camilleri, Commented Dec 6, 2016 at 18:18

fjs6 · Accepted Answer · 2010-03-17 00:37:32Z

10

This works

eval  "lambda { " + code_string + " }"

I just don't know why this one does and the other does not.

answered Mar 17, 2010 at 0:37

fjs6

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2 Comments

Fred Over a year ago

Calling lambda on an eval argument results in a Proc object with the eval call 'inside' the Proc object. The resulting Proc object doesn't take an argument, as the expression eval(code_string) doesn't take an argument. When you call the Proc object, it evals the code_string! The eval of the string "lambda { " + code_string + " }" gives a Proc object that is expecting an argument, and returns 2*argument.

Chuck Over a year ago

Also, it's more idiomatic (and more efficient to boot) to use string interpolation, so it would be: eval "lambda {#{code_string}}". Concatenating several strings with + is rarely done in Ruby.

Weston Ganger · Accepted Answer · 2016-08-26 15:41:23Z

4

eval "lambda {#{code_string}}"

answered Aug 26, 2016 at 15:41

Weston Ganger

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