Merge pull request #79 from phel-lang/add-blog-loop

Add loop blog post

content/blog/loop.md

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+title = "Loop"
+date = "2024-05-18"
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+
+Many functional programming models prefer to express repetition by recursive function calls.
+
+In Phel's iteration process, there is a highly functional and convenient `for` macro, but there is also a `loop` special form that performs more primitive loop processing.
+
+[Loop Document | Control flow | The Phel Language](https://phel-lang.org/documentation/control-flow/#loop)
+
+Phel's `loop` allows you to write repetitive processing just like a recursive function.
+
+```phel
+(loop [sum 0
+       cnt 10]
+  (if (= cnt 0)
+    sum
+    (recur (+ cnt sum) (dec cnt))))
+=> 55
+```
+
+When the same function is written as a recursive function, it looks like this:
+
+```phel
+(defn my-sum-to-n [sum cnt]
+  (if (= cnt 0)
+    sum
+    (my-sum-to-n (+ cnt sum) (dec cnt))))
+(my-sum-to-n 0 10)
+=> 55
+```
+
+There are some differences between the `loop` format and the recursive function format.
+When calling as recursion, the `loop` format specifies `recur`, but the recursive function format specifies its own function name `my-sum-to-n`.
+For everything else, you can see that you can use `loop` to write iterations in the same way you would write recursive functions.
+
+However, there is one more thing to keep in mind.
+The recursive structure of `loop` must be tail recursive.
+This means that `recur` can only be placed at the location where it is evaluated last in the iteration process within `loop`.
+If you try to place `recur` in any other location, the following error will be displayed.
+
+```
+ERROR: Can't call 'recur here
+```
+
+If this error is displayed, please check whether the recursive structure of `loop` is tail recursive.