post on side effects and partial application

_posts/2011-06-19-side-effects-and-partial-application.markdown

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+---
+layout: post
+title: "F#: Use care mixing side-effects and partial application"
+date: 2011-06-19
+---
+
+Here's a lesson I learned while writing
+[llvm-fs](https://github.com/keithshep/llvm-fs). Do you see the problem in my
+buggy_iprintfn function?
+
+{% highlight ocaml %}
+let buggy_iprintfn depth fmt =
+    for i = 0 to depth - 1 do
+        printf "\t"
+    printfn fmt
+{% endhighlight %}
+
+It works fine if you apply all arguments each time you call it like:
+
+{% highlight ocaml %}
+buggy_iprintfn 2 "hey %s, what's up" "billy"
+buggy_iprintfn 2 "hey %s, what's up" "bob"
+buggy_iprintfn 2 "hey %s, what's up" "billy-bob"
+{% endhighlight %}
+
+Output:
+<pre>
+        hey billy, what's up
+        hey bob, what's up
+        hey billy-bob, what's up
+</pre>
+
+But fails to indent anything after the first name if you call it like:
+
+{% highlight ocaml %}
+List.iter (buggy_iprintfn 2 "hey %s, what's up") ["billy"; "bob"; "billy-bob"]
+{% endhighlight %}
+
+Output:
+<pre>
+        hey billy, what's up
+hey bob, what's up
+hey billy-bob, what's up
+</pre>
+
+The problem occurs because buggy_iprintfn returns a
+[function value](http://msdn.microsoft.com/en-us/library/dd233158.aspx) and the
+(print "\t") side effects occur before the (printfn fmt) function value
+is returned. As a side-note this bug is a good argument for the haskell approach
+of isolating side effects to [monads](http://www.haskell.org/haskellwiki/Monad)
+where this bug cannot occur. The iprintfn function
+below fixes the bug by moving all side effects into printIndented which will not
+execute until all arguments are applied to the returned function value.
+
+{% highlight ocaml %}
+let iprintfn depth fmt =
+    let printIndented s =
+        for i = 0 to depth - 1 do
+            printf "\t"
+        printfn "%s" s
+    Printf.ksprintf printIndented fmt
+
+List.iter (iprintfn 2 "hey %s, what's up") ["billy"; "bob"; "billy-bob"]
+{% endhighlight %}
+
+Output:
+<pre>
+        hey billy, what's up
+        hey bob, what's up
+        hey billy-bob, what's up
+</pre>
+