let

I want to make a local instance of a Java Scanner class in a clojure program. Why does this not work: ; gives me: count not supported on this type: Symbol (let s (new Scanner "a b c")) but it will let me create a global instance like this: (def s (new Scanner "a b c")) I was under the impression that the only difference was scope, but apparently not. What is the difference between let and def ? Rayne The problem is that your use of let is wrong. Let works like this: (let [identifier (expr)]) So your example should be something like this: (let [s (Scanner. "a b c")] (exprs)) You can only use

This question already has an answer here: Do let statements create properties on the global object? 5 answers I used to check if a global var has been defined with: if (window['myvar']==null) ... or if (window.myvar==null) ... It works with var myvar Now that I am trying to switch to let, this does not work anymore. var myvar='a'; console.log(window.myvar); // gives me a let mylet='b'; console.log(window.mylet); // gives me undefined Question: With a global let , is there any place I can look if something has been defined like I could with var from the window object? More generally : Is var

Regardless of functional differences, does using the new keywords 'let' and 'const' have any generalized or specific impact on performance relative to 'var'? After running the program: function timeit(f, N, S) { var start, timeTaken; var stats = {min: 1e50, max: 0, N: 0, sum: 0, sqsum: 0}; var i; for (i = 0; i < S; ++i) { start = Date.now(); f(N); timeTaken = Date.now() - start; stats.min = Math.min(timeTaken, stats.min); stats.max = Math.max(timeTaken, stats.max); stats.sum += timeTaken; stats.sqsum += timeTaken * timeTaken; stats.N++ } var mean = stats.sum / stats.N; var sqmean = stats.sqsum

How can I rewrite this linq query to Entity on with lambda expression? I want to use let keyword or an equivalent in my lambda expression. var results = from store in Stores let AveragePrice = store.Sales.Average(s => s.Price) where AveragePrice < 500 && AveragePrice > 250 For some similar questions like what is commented under my question, it's suggested to .Select(store=> new { AveragePrice = store.Sales.Average(s => s.Price), store}) which will calculate AveragePrice for each item, while in Query style I mentioned, let expression prevents to calculate average many times. So, you can use the

I understand why var takes that name - it is variable, const - it is a constant, but what is the meaning behind the name for let , which scopes to the current block? Let it be? Let is a mathematical statement that was adopted by early programming languages like Scheme and Basic. Variables are considered low level entities not suitable for higher levels of abstraction, thus the desire of many language designers to introduce similar but more powerful concepts like in Clojure, F#, Scala, where let might mean a value, or a variable that can be assigned, but not changed , which in turn lets the