<!DOCTYPE html> <html lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <meta name="generator" content="AsciiDoc 8.6.8"> <title>Printf</title> <link rel="stylesheet" href="./asciidoc.css" type="text/css"> <link rel="stylesheet" href="./pygments.css" type="text/css"> <script type="text/javascript" src="./asciidoc.js"></script> <script type="text/javascript"> /*<![CDATA[*/ asciidoc.install(); /*]]>*/ </script> <link rel="stylesheet" href="./mlton.css" type="text/css"/> </head> <body class="article"> <div id="banner"> <div id="banner-home"> <a href="./Home">MLton 20130715</a> </div> </div> <div id="header"> <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 <span class="monospaced">printf</span> 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 <span class="monospaced">printf</span> and <span class="monospaced">fprintf</span>, along with format specifiers for booleans, integers, and reals.</p></div> <div class="listingblock"> <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> <span class="w"> </span><span class="k">struct</span><span class="w"></span> <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">=></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> <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> <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">=></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> <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> <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">=></span><span class="w"></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">=></span><span class="w"></span> <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">=></span><span class="w"></span> <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">=></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> <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">=></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="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">=></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> <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">=></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> <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">=></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> <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">=></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> <span class="w"> </span><span class="k">end</span><span class="w"></span> </pre></div></div></div> <div class="paragraph"><p>Here’s an example use.</p></div> <div class="listingblock"> <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">"Int="</span><span class="n">I`</span><span class="s">" Bool="</span><span class="n">B`</span><span class="s">" Real="</span><span class="n">R`</span><span class="s">"</span><span class="se">\n</span><span class="s">"</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> </pre></div></div></div> <div class="paragraph"><p>This prints the following.</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>Int=1 Bool=false Real=2.0</pre> </div></div> <div class="paragraph"><p>In general, a use of <span class="monospaced">printf</span> looks like</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>printf <spec1> ... <specn> $ <arg1> ... <argm></pre> </div></div> <div class="paragraph"><p>where each <span class="monospaced"><speci></span> is either a specifier like <span class="monospaced">B</span>, <span class="monospaced">I</span>, or <span class="monospaced">R</span>, or is an inline string, like <span class="monospaced">`"foo"</span>. A backtick (<span class="monospaced">`</span>) must precede each inline string. Each <span class="monospaced"><argi></span> must be of the appropriate type for the corresponding specifier.</p></div> <div class="paragraph"><p>SML <span class="monospaced">printf</span> is more powerful than its C counterpart in a number of ways. In particular, the function produced by <span class="monospaced">printf</span> 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"><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">-></span><span class="w"> </span><span class="n">bool</span><span class="w"> </span><span class="p">-></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">"Int="</span><span class="n">I`</span><span class="s">" Bool="</span><span class="n">B`</span><span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="w"> </span><span class="n">$</span><span class="w"></span> <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> <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> </pre></div></div></div> <div class="paragraph"><p>The definition of <span class="monospaced">printf</span> 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 monospaced"> <pre>val f: int -> bool -> unit = printf `"Int="I`" Bool="B`"\n" $ 13 val () = f true val () = f false</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 monospaced"> <pre>val C = fn z => spec Char.toString z val S = fn z => spec (fn s => s) z</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 monospaced"> <pre>val I2 = fn z => spec (fn (i, j) => concat ["(", Int.toString i, ", ", Int.toString j, ")"]) z</pre> </div></div> <div class="paragraph"><p>Here’s an example use.</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>val () = printf `"Test "I2`" a string "S`"\n" $ (1, 2) "hello"</pre> </div></div> </div> </div> <div class="sect1"> <h2 id="_printf_via_a_href_fold_fold_a">Printf via <a href="Fold">Fold</a></h2> <div class="sectionbody"> <div class="paragraph"><p><span class="monospaced">printf</span> 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 <span class="monospaced">Printf</span> structure in terms of fold. The structure is equivalent to the above one, except that it uses the standard <span class="monospaced">$</span> instead of a specialized one.</p></div> <div class="listingblock"> <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> <span class="w"> </span><span class="k">struct</span><span class="w"></span> <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> <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">=></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">=></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> <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">=></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> <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> <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">=></span><span class="w"></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">=></span><span class="w"></span> <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">=></span><span class="w"></span> <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">=></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> <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> <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">=></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">=></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">=></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> <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">=></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">=></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> <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">=></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> <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">=></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> <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">=></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> <span class="w"> </span><span class="k">end</span><span class="w"></span> </pre></div></div></div> <div class="paragraph"><p>Viewing <span class="monospaced">printf</span> 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 monospaced"> <pre>val IB = fn u => Fold.fold u `"Int="I`" Bool="B val () = printf IB`" "IB`"\n" $ 1 true 3 false</pre> </div></div> <div class="paragraph"><p>One can even parametrize over partial format strings.</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>fun XB X = fn u => Fold.fold u `"X="X`" Bool="B val () = printf (XB I)`" "(XB R)`"\n" $ 1 true 2.0 false</pre> </div></div> </div> </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></div> </div> </div> </div> <div id="footnotes"><hr></div> <div id="footer"> <div id="footer-text"> </div> <div id="footer-badges"> </div> </div> </body> </html>