doc/guide/localhost/Printf

   1 <!DOCTYPE html>
   2 <html lang="en">
   3 <head>
   4 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
   5 <meta name="generator" content="AsciiDoc 8.6.9">
   6 <title>Printf</title>
   7 <link rel="stylesheet" href="./asciidoc.css" type="text/css">
   8 <link rel="stylesheet" href="./pygments.css" type="text/css">
   9
  10
  11 <script type="text/javascript" src="./asciidoc.js"></script>
  12 <script type="text/javascript">
  13 /*<![CDATA[*/
  14 asciidoc.install();
  15 /*]]>*/
  16 </script>
  17 <link rel="stylesheet" href="./mlton.css" type="text/css">
  18 </head>
  19 <body class="article">
  20 <div id="banner">
  21 <div id="banner-home">
  22 <a href="./Home">MLton 20180207</a>
  23 </div>
  24 </div>
  25 <div id="header">
  26 <h1>Printf</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p>Programmers coming from C or Java often ask if
  32 <a href="StandardML">Standard ML</a> has a <span class="monospaced">printf</span> function.  It does not.
  33 However, it is possible to implement your own version with only a few
  34 lines of code.</p></div>
  35 <div class="paragraph"><p>Here is a definition for <span class="monospaced">printf</span> and <span class="monospaced">fprintf</span>, along with format
  36 specifiers for booleans, integers, and reals.</p></div>
  37 <div class="listingblock">
  38 <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Printf</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
  39 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
  40 <span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">$</span><span class="w"> </span><span class="p">(_,</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="p">())</span><span class="w"> </span><span class="n">ignore</span><span class="w"></span>
  41 <span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">fprintf</span><span class="w"> </span><span class="n">out</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="n">out</span><span class="p">,</span><span class="w"> </span><span class="n">id</span><span class="p">)</span><span class="w"></span>
  42 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">printf</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">fprintf</span><span class="w"> </span><span class="n">TextIO</span><span class="p">.</span><span class="n">stdOut</span><span class="w"> </span><span class="n">z</span><span class="w"></span>
  43 <span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">one</span><span class="w"> </span><span class="p">((</span><span class="n">out</span><span class="p">,</span><span class="w"> </span><span class="n">f</span><span class="p">),</span><span class="w"> </span><span class="n">make</span><span class="p">)</span><span class="w"> </span><span class="n">g</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
  44 <span class="w">         </span><span class="n">g</span><span class="w"> </span><span class="p">(</span><span class="n">out</span><span class="p">,</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"></span>
  45 <span class="w">            </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"></span>
  46 <span class="w">               </span><span class="n">make</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"></span>
  47 <span class="w">                     </span><span class="n">r</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">(</span><span class="n">p</span><span class="w"> </span><span class="p">();</span><span class="w"> </span><span class="n">TextIO</span><span class="p">.</span><span class="n">output</span><span class="w"> </span><span class="p">(</span><span class="n">out</span><span class="p">,</span><span class="w"> </span><span class="n">s</span><span class="p">))))))</span><span class="w"></span>
  48 <span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">`</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">one</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="n">s</span><span class="p">)</span><span class="w"></span>
  49 <span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">spec</span><span class="w"> </span><span class="n">to</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">one</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">to</span><span class="p">)</span><span class="w"></span>
  50 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">spec</span><span class="w"> </span><span class="n">Bool</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">z</span><span class="w"></span>
  51 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">I</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">spec</span><span class="w"> </span><span class="n">Int</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">z</span><span class="w"></span>
  52 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">R</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">spec</span><span class="w"> </span><span class="n">Real</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">z</span><span class="w"></span>
  53 <span class="w">   </span><span class="k">end</span><span class="w"></span>
  54 </pre></div></div></div>
  55 <div class="paragraph"><p>Here&#8217;s an example use.</p></div>
  56 <div class="listingblock">
  57 <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">printf</span><span class="w"> </span><span class="n">`</span><span class="s">&quot;Int=&quot;</span><span class="n">I`</span><span class="s">&quot;  Bool=&quot;</span><span class="n">B`</span><span class="s">&quot;  Real=&quot;</span><span class="n">R`</span><span class="s">&quot;</span><span class="se">\n</span><span class="s">&quot;</span><span class="w"> </span><span class="n">$</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="n">false</span><span class="w"> </span><span class="mf">2.0</span><span class="w"></span>
  58 </pre></div></div></div>
  59 <div class="paragraph"><p>This prints the following.</p></div>
  60 <div class="listingblock">
  61 <div class="content monospaced">
  62 <pre>Int=1  Bool=false  Real=2.0</pre>
  63 </div></div>
  64 <div class="paragraph"><p>In general, a use of <span class="monospaced">printf</span> looks like</p></div>
  65 <div class="listingblock">
  66 <div class="content monospaced">
  67 <pre>printf &lt;spec1&gt; ... &lt;specn&gt; $ &lt;arg1&gt; ... &lt;argm&gt;</pre>
  68 </div></div>
  69 <div class="paragraph"><p>where each <span class="monospaced">&lt;speci&gt;</span> is either a specifier like <span class="monospaced">B</span>, <span class="monospaced">I</span>, or <span class="monospaced">R</span>, or
  70 is an inline string, like <span class="monospaced">&grave;"foo"</span>.  A backtick (<span class="monospaced">&grave;</span>)
  71 must precede each inline string.  Each <span class="monospaced">&lt;argi&gt;</span> must be of the
  72 appropriate type for the corresponding specifier.</p></div>
  73 <div class="paragraph"><p>SML <span class="monospaced">printf</span> is more powerful than its C counterpart in a number of
  74 ways.  In particular, the function produced by <span class="monospaced">printf</span> is a perfectly
  75 ordinary SML function, and can be passed around, used multiple times,
  76 etc.  For example:</p></div>
  77 <div class="listingblock">
  78 <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">f</span><span class="p">:</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">bool</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">printf</span><span class="w"> </span><span class="n">`</span><span class="s">&quot;Int=&quot;</span><span class="n">I`</span><span class="s">&quot;  Bool=&quot;</span><span class="n">B`</span><span class="s">&quot;</span><span class="se">\n</span><span class="s">&quot;</span><span class="w"> </span><span class="n">$</span><span class="w"></span>
  79 <span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="n">true</span><span class="w"></span>
  80 <span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="n">false</span><span class="w"></span>
  81 </pre></div></div></div>
  82 <div class="paragraph"><p>The definition of <span class="monospaced">printf</span> is even careful to not print anything until
  83 it is fully applied.  So, examples like the following will work as
  84 expected.</p></div>
  85 <div class="listingblock">
  86 <div class="content monospaced">
  87 <pre>val f: int -&gt; bool -&gt; unit = printf `"Int="I`"  Bool="B`"\n" $ 13
  88 val () = f true
  89 val () = f false</pre>
  90 </div></div>
  91 <div class="paragraph"><p>It is also easy to define new format specifiers.  For example, suppose
  92 we wanted format specifiers for characters and strings.</p></div>
  93 <div class="listingblock">
  94 <div class="content monospaced">
  95 <pre>val C = fn z =&gt; spec Char.toString z
  96 val S = fn z =&gt; spec (fn s =&gt; s) z</pre>
  97 </div></div>
  98 <div class="paragraph"><p>One can define format specifiers for more complex types, e.g. pairs of
  99 integers.</p></div>
 100 <div class="listingblock">
 101 <div class="content monospaced">
 102 <pre>val I2 =
 103    fn z =&gt;
 104    spec (fn (i, j) =&gt;
 105          concat ["(", Int.toString i, ", ", Int.toString j, ")"])
 106    z</pre>
 107 </div></div>
 108 <div class="paragraph"><p>Here&#8217;s an example use.</p></div>
 109 <div class="listingblock">
 110 <div class="content monospaced">
 111 <pre>val () = printf `"Test "I2`"  a string "S`"\n" $ (1, 2) "hello"</pre>
 112 </div></div>
 113 </div>
 114 </div>
 115 <div class="sect1">
 116 <h2 id="_printf_via_a_href_fold_fold_a">Printf via <a href="Fold">Fold</a></h2>
 117 <div class="sectionbody">
 118 <div class="paragraph"><p><span class="monospaced">printf</span> is best viewed as a special case of variable-argument
 119 <a href="Fold">Fold</a> that inductively builds a function as it processes its
 120 arguments.  Here is the definition of a <span class="monospaced">Printf</span> structure in terms of
 121 fold.  The structure is equivalent to the above one, except that it
 122 uses the standard <span class="monospaced">$</span> instead of a specialized one.</p></div>
 123 <div class="listingblock">
 124 <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Printf</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 125 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
 126 <span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">fprintf</span><span class="w"> </span><span class="n">out</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 127 <span class="w">         </span><span class="n">Fold</span><span class="p">.</span><span class="n">fold</span><span class="w"> </span><span class="p">((</span><span class="n">out</span><span class="p">,</span><span class="w"> </span><span class="n">id</span><span class="p">),</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">(_,</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="p">())</span><span class="w"> </span><span class="n">ignore</span><span class="p">)</span><span class="w"></span>
 128
 129 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">printf</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">fprintf</span><span class="w"> </span><span class="n">TextIO</span><span class="p">.</span><span class="n">stdOut</span><span class="w"> </span><span class="n">z</span><span class="w"></span>
 130
 131 <span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">one</span><span class="w"> </span><span class="p">((</span><span class="n">out</span><span class="p">,</span><span class="w"> </span><span class="n">f</span><span class="p">),</span><span class="w"> </span><span class="n">make</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 132 <span class="w">         </span><span class="p">(</span><span class="n">out</span><span class="p">,</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"></span>
 133 <span class="w">          </span><span class="n">f</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"></span>
 134 <span class="w">             </span><span class="n">make</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"></span>
 135 <span class="w">                   </span><span class="n">r</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">(</span><span class="n">p</span><span class="w"> </span><span class="p">();</span><span class="w"> </span><span class="n">TextIO</span><span class="p">.</span><span class="n">output</span><span class="w"> </span><span class="p">(</span><span class="n">out</span><span class="p">,</span><span class="w"> </span><span class="n">s</span><span class="p">))))))</span><span class="w"></span>
 136
 137 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">`</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 138 <span class="w">         </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">Fold</span><span class="p">.</span><span class="n">step1</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">(</span><span class="n">s</span><span class="p">,</span><span class="w"> </span><span class="n">x</span><span class="p">)</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">one</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="n">s</span><span class="p">))</span><span class="w"> </span><span class="n">z</span><span class="w"></span>
 139
 140 <span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">spec</span><span class="w"> </span><span class="n">to</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Fold</span><span class="p">.</span><span class="n">step0</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">one</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">to</span><span class="p">))</span><span class="w"></span>
 141
 142 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">spec</span><span class="w"> </span><span class="n">Bool</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">z</span><span class="w"></span>
 143 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">I</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">spec</span><span class="w"> </span><span class="n">Int</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">z</span><span class="w"></span>
 144 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">R</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">z</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">spec</span><span class="w"> </span><span class="n">Real</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="n">z</span><span class="w"></span>
 145 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 146 </pre></div></div></div>
 147 <div class="paragraph"><p>Viewing <span class="monospaced">printf</span> as a fold opens up a number of possibilities.  For
 148 example, one can name parts of format strings using the fold idiom for
 149 naming sequences of steps.</p></div>
 150 <div class="listingblock">
 151 <div class="content monospaced">
 152 <pre>val IB = fn u =&gt; Fold.fold u `"Int="I`" Bool="B
 153 val () = printf IB`"  "IB`"\n" $ 1 true 3 false</pre>
 154 </div></div>
 155 <div class="paragraph"><p>One can even parametrize over partial format strings.</p></div>
 156 <div class="listingblock">
 157 <div class="content monospaced">
 158 <pre>fun XB X = fn u =&gt; Fold.fold u `"X="X`" Bool="B
 159 val () = printf (XB I)`"  "(XB R)`"\n" $ 1 true 2.0 false</pre>
 160 </div></div>
 161 </div>
 162 </div>
 163 <div class="sect1">
 164 <h2 id="_also_see">Also see</h2>
 165 <div class="sectionbody">
 166 <div class="ulist"><ul>
 167 <li>
 168 <p>
 169 <a href="PrintfGentle">PrintfGentle</a>
 170 </p>
 171 </li>
 172 <li>
 173 <p>
 174 <a href="References#Danvy98"> Functional Unparsing</a>
 175 </p>
 176 </li>
 177 </ul></div>
 178 </div>
 179 </div>
 180 </div>
 181 <div id="footnotes"><hr></div>
 182 <div id="footer">
 183 <div id="footer-text">
 184 </div>
 185 <div id="footer-badges">
 186 </div>
 187 </div>
 188 </body>
 189 </html>