I understand that in general $ is a) right-associative and b) lowest-priority. But if so shouldn't these two be roughly the same? λ take (succ 10) $ cycle "hello world" "hello world" But not this? λ take $ succ 10 $ cycle "hello world" <interactive>:20:8: error: • No instance for (Enum ([Char] -> Int)) arising from a use of ‘succ’ (maybe you haven't applied a function to enough arguments?) • In the expression: succ 10 In the second argument of ‘($)’, namely ‘succ 10 $ cycle "hello world"’ In the expression: take $ succ 10 $ cycle "hello world" <interactive>:20:13: error: • No instance for (Num ([Char] -> Int)) arising from the literal ‘10’ (maybe you haven't applied a function to enough arguments?) • In the first argument of ‘succ’, namely ‘10’ In the expression: succ 10 In the second argument of ‘($)’, namely ‘succ 10 $ cycle "hello world"’ _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners |
Because the second one is: take (succ 10 (cycle “hello world”)) On Thu, Aug 20, 2020 at 12:03 Josh Friedlander <[hidden email]> wrote:
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In reply to this post by Josh Friedlander
Hello Josh,
il 20 agosto 2020 alle 22:02 josh friedlander ha scritto: > i understand that in general $ is a) right-associative and b) > lowest-priority. but if so shouldn't these two be roughly the same? > > λ take (succ 10) $ cycle "hello world" > "hello world" > > But not this? > λ take $ succ 10 $ cycle "hello world" > > […] λ> :info ($) ($) :: (a -> b) -> a -> b -- Defined in ‘GHC.Base’ infixr 0 $ So, since `$` is right associative, the expression take $ succ 10 $ cycle "hello world" becomes take (succ 10 (cycle "hello world")) `cycle "hello world"` makes sense, `succ 10` makes sense, `succ 10 anotherArgument` does not. Even `take someStuff` is probably not what you want, since take is usually invoked with two arguments. A useful intuition when you see ($) is `it will evaluate everything on the right of it first`. This way, `not $ xx yy zz` looks right, `take $ xx aa yy qq` less so. Does this help? —F _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners |
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