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Please find the code in the image below. 1. Assign the returned value of a function, which throws an error actually, to the variable 'withLet' that declared by using keyword 'let'. 2. call 'withLet', an error occured: 'withLet is not defined'. 3. try to assert 'withLet' using 'let', an error shows that 'withLet' has already been declared. But the paradox is not exist for 'var' (Please find in the following image). I'm curious about what caused the different behaviors between these two situations. It's quite wired that 'not defined' an 'already been declared' describe a same variable. let

If I say let 5 = 10 , why does 5 + 1 return 6 instead of 11 ? When you say let 5 = 10 it's not a redefinition of 5, it's a pattern matching, the same which occurs when you say foo 5 = undefined ... foo 10 ... The pattern simply fails if it's ever matched. In let-expressions the match is lazy. This means the match is only being done when a variable bound by it is evaluated. This allows us to write things like let foo = undefined in 10 In your expression, no variable is bound, so the pattern is never matched. Arguably such patterns with no variables make no sense in let-bindings and should be

I'd appreciate is someone could point to docs on what "let" does in GHCi, or failing that, explain it convincingly :-). So far as I can tell, "let" (without "in") is not part of the Haskell language per se, and on the other hand, it doesn't appear to be a GHCI command either, as it's not prefixed by colon. sinan While programming in GHCi, you're like programming in the IO monad with do syntax, so for example you can directly execute an IO action, or use monadic bind syntax like r <- someIOFun . let is also a part of do so you can also use this. I think it's being desugared into let .. in <rest

I am new to Haskell and I am very confused by Where vs. Let . They both seem to provide a similar purpose. I have read a few comparisons between Where vs. Let but I am having trouble discerning when to use each. Could someone please provide some context or perhaps a few examples that demonstrate when to use one over the other? Where vs. Let A where clause can only be defined at the level of a function definition. Usually, that is identical to the scope of let definition. The only difference is when guards are being used . The scope of the where clause extends over all guards. In contrast, the

In the following code, the last phrase I can put an in in front. Will it change anything? Another question: If I decide to put in in front of the last phrase, do I need to indent it? I tried without indenting and hugs complains Last generator in do {...} must be an expression import Data.Char groupsOf _ [] = [] groupsOf n xs = take n xs : groupsOf n ( tail xs ) problem_8 x = maximum . map product . groupsOf 5 $ x main = do t <- readFile "p8.log" let digits = map digitToInt $concat $ lines t print $ problem_8 digits Edit Ok, so people don't seem to understand what I'm saying. Let me rephrase:

In Clojure, (def x 3) (eval '(prn x)) prints 3, whereas (let [y 3] (eval '(prn y))) and (binding [z 3] (eval '(prn z))) generate an 'Unable to resolve var' exception. According to http://clojure.org/evaluation , eval , load-string , etc generate temporary namespaces to evaluate their contents. Therefore, I'd expect neither of the above code samples to work, since (def x 3) is done in my current namespace, not the one created by eval . Why does the first code sample work and not the last two? How can I eval a form with bound variables without using def ? Thanks! danlei 1.: The reason this doesn