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20241230+git20251029+dfsg-5
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<h1>Printf</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>Programmers coming from C or Java often ask if
<a href="StandardML">Standard ML</a> has a <code>printf</code> function.  It does not.
However, it is possible to implement your own version with only a few
lines of code.</p>
</div>
<div class="paragraph">
<p>Here is a definition for <code>printf</code> and <code>fprintf</code>, along with format
specifiers for booleans, integers, and reals.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sml">structure Printf =
   struct
      fun $ (_, f) = f (fn p =&gt; p ()) ignore
      fun fprintf out f = f (out, id)
      val printf = fn z =&gt; fprintf TextIO.stdOut z
      fun one ((out, f), make) g =
         g (out, fn r =&gt;
            f (fn p =&gt;
               make (fn s =&gt;
                     r (fn () =&gt; (p (); TextIO.output (out, s))))))
      fun ` x s = one (x, fn f =&gt; f s)
      fun spec to x = one (x, fn f =&gt; f o to)
      val B = fn z =&gt; spec Bool.toString z
      val I = fn z =&gt; spec Int.toString z
      val R = fn z =&gt; spec Real.toString z
   end</code></pre>
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</div>
<div class="paragraph">
<p>Here&#8217;s an example use.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sml">val () = printf `"Int="I`"  Bool="B`"  Real="R`"\n" $ 1 false 2.0</code></pre>
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<div class="paragraph">
<p>This prints the following.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>Int=1  Bool=false  Real=2.0</pre>
</div>
</div>
<div class="paragraph">
<p>In general, a use of <code>printf</code> looks like</p>
</div>
<div class="listingblock">
<div class="content">
<pre>printf &lt;spec1&gt; ... &lt;specn&gt; $ &lt;arg1&gt; ... &lt;argm&gt;</pre>
</div>
</div>
<div class="paragraph">
<p>where each <code>&lt;speci&gt;</code> is either a specifier like <code>B</code>, <code>I</code>, or <code>R</code>, or
is an inline string, like <code>`"foo"</code>.  A backtick (<code>`</code>)
must precede each inline string.  Each <code>&lt;argi&gt;</code> must be of the
appropriate type for the corresponding specifier.</p>
</div>
<div class="paragraph">
<p>SML <code>printf</code> is more powerful than its C counterpart in a number of
ways.  In particular, the function produced by <code>printf</code> is a perfectly
ordinary SML function, and can be passed around, used multiple times,
etc.  For example:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sml">val f: int -&gt; bool -&gt; unit = printf `"Int="I`"  Bool="B`"\n" $
val () = f 1 true
val () = f 2 false</code></pre>
</div>
</div>
<div class="paragraph">
<p>The definition of <code>printf</code> is even careful to not print anything until
it is fully applied.  So, examples like the following will work as
expected.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sml">val f: int -&gt; bool -&gt; unit = printf `"Int="I`"  Bool="B`"\n" $ 13
val () = f true
val () = f false</code></pre>
</div>
</div>
<div class="paragraph">
<p>It is also easy to define new format specifiers.  For example, suppose
we wanted format specifiers for characters and strings.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sml">val C = fn z =&gt; spec Char.toString z
val S = fn z =&gt; spec (fn s =&gt; s) z</code></pre>
</div>
</div>
<div class="paragraph">
<p>One can define format specifiers for more complex types, e.g. pairs of
integers.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sml">val I2 =
   fn z =&gt;
   spec (fn (i, j) =&gt;
         concat ["(", Int.toString i, ", ", Int.toString j, ")"])
   z</code></pre>
</div>
</div>
<div class="paragraph">
<p>Here&#8217;s an example use.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sml">val () = printf `"Test "I2`"  a string "S`"\n" $ (1, 2) "hello"</code></pre>
</div>
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</div>
<div class="sect1">
<h2 id="_printf_via_fold">Printf via <a href="Fold">Fold</a></h2>
<div class="sectionbody">
<div class="paragraph">
<p><code>printf</code> is best viewed as a special case of variable-argument
<a href="Fold">Fold</a> that inductively builds a function as it processes its
arguments.  Here is the definition of a <code>Printf</code> structure in terms of
fold.  The structure is equivalent to the above one, except that it
uses the standard <code>$</code> instead of a specialized one.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sml">structure Printf =
   struct
      fun fprintf out =
         Fold.fold ((out, id), fn (_, f) =&gt; f (fn p =&gt; p ()) ignore)

      val printf = fn z =&gt; fprintf TextIO.stdOut z

      fun one ((out, f), make) =
         (out, fn r =&gt;
          f (fn p =&gt;
             make (fn s =&gt;
                   r (fn () =&gt; (p (); TextIO.output (out, s))))))

      val ` =
         fn z =&gt; Fold.step1 (fn (s, x) =&gt; one (x, fn f =&gt; f s)) z

      fun spec to = Fold.step0 (fn x =&gt; one (x, fn f =&gt; f o to))

      val B = fn z =&gt; spec Bool.toString z
      val I = fn z =&gt; spec Int.toString z
      val R = fn z =&gt; spec Real.toString z
   end</code></pre>
</div>
</div>
<div class="paragraph">
<p>Viewing <code>printf</code> as a fold opens up a number of possibilities.  For
example, one can name parts of format strings using the fold idiom for
naming sequences of steps.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sml">val IB = fn u =&gt; Fold.fold u `"Int="I`" Bool="B
val () = printf IB`"  "IB`"\n" $ 1 true 3 false</code></pre>
</div>
</div>
<div class="paragraph">
<p>One can even parametrize over partial format strings.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="sml">fun XB X = fn u =&gt; Fold.fold u `"X="X`" Bool="B
val () = printf (XB I)`"  "(XB R)`"\n" $ 1 true 2.0 false</code></pre>
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</div>
<div class="sect1">
<h2 id="_also_see">Also see</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p><a href="PrintfGentle">PrintfGentle</a></p>
</li>
<li>
<p><a href="References#Danvy98">Functional Unparsing</a></p>
</li>
</ul>
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