Commit | Line | Data |
---|---|---|
7f918cf1 CE |
1 | <!DOCTYPE html>\r |
2 | <html lang="en">\r | |
3 | <head>\r | |
4 | <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">\r | |
5 | <meta name="generator" content="AsciiDoc 8.6.9">\r | |
6 | <title>Printf</title>\r | |
7 | <link rel="stylesheet" href="./asciidoc.css" type="text/css">\r | |
8 | <link rel="stylesheet" href="./pygments.css" type="text/css">\r | |
9 | \r | |
10 | \r | |
11 | <script type="text/javascript" src="./asciidoc.js"></script>\r | |
12 | <script type="text/javascript">\r | |
13 | /*<![CDATA[*/\r | |
14 | asciidoc.install();\r | |
15 | /*]]>*/\r | |
16 | </script>\r | |
17 | <link rel="stylesheet" href="./mlton.css" type="text/css">\r | |
18 | </head>\r | |
19 | <body class="article">\r | |
20 | <div id="banner">\r | |
21 | <div id="banner-home">\r | |
22 | <a href="./Home">MLton 20180207</a>\r | |
23 | </div>\r | |
24 | </div>\r | |
25 | <div id="header">\r | |
26 | <h1>Printf</h1>\r | |
27 | </div>\r | |
28 | <div id="content">\r | |
29 | <div id="preamble">\r | |
30 | <div class="sectionbody">\r | |
31 | <div class="paragraph"><p>Programmers coming from C or Java often ask if\r | |
32 | <a href="StandardML">Standard ML</a> has a <span class="monospaced">printf</span> function. It does not.\r | |
33 | However, it is possible to implement your own version with only a few\r | |
34 | lines of code.</p></div>\r | |
35 | <div class="paragraph"><p>Here is a definition for <span class="monospaced">printf</span> and <span class="monospaced">fprintf</span>, along with format\r | |
36 | specifiers for booleans, integers, and reals.</p></div>\r | |
37 | <div class="listingblock">\r | |
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>\r | |
39 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
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">=></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>\r | |
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>\r | |
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">=></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>\r | |
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>\r | |
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">=></span><span class="w"></span>\r | |
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">=></span><span class="w"></span>\r | |
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">=></span><span class="w"></span>\r | |
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">=></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>\r | |
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">=></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>\r | |
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">=></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>\r | |
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">=></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>\r | |
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">=></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>\r | |
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">=></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>\r | |
53 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
54 | </pre></div></div></div>\r | |
55 | <div class="paragraph"><p>Here’s an example use.</p></div>\r | |
56 | <div class="listingblock">\r | |
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">"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>\r | |
58 | </pre></div></div></div>\r | |
59 | <div class="paragraph"><p>This prints the following.</p></div>\r | |
60 | <div class="listingblock">\r | |
61 | <div class="content monospaced">\r | |
62 | <pre>Int=1 Bool=false Real=2.0</pre>\r | |
63 | </div></div>\r | |
64 | <div class="paragraph"><p>In general, a use of <span class="monospaced">printf</span> looks like</p></div>\r | |
65 | <div class="listingblock">\r | |
66 | <div class="content monospaced">\r | |
67 | <pre>printf <spec1> ... <specn> $ <arg1> ... <argm></pre>\r | |
68 | </div></div>\r | |
69 | <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\r | |
70 | is an inline string, like <span class="monospaced">`"foo"</span>. A backtick (<span class="monospaced">`</span>)\r | |
71 | must precede each inline string. Each <span class="monospaced"><argi></span> must be of the\r | |
72 | appropriate type for the corresponding specifier.</p></div>\r | |
73 | <div class="paragraph"><p>SML <span class="monospaced">printf</span> is more powerful than its C counterpart in a number of\r | |
74 | ways. In particular, the function produced by <span class="monospaced">printf</span> is a perfectly\r | |
75 | ordinary SML function, and can be passed around, used multiple times,\r | |
76 | etc. For example:</p></div>\r | |
77 | <div class="listingblock">\r | |
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">-></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>\r | |
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>\r | |
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>\r | |
81 | </pre></div></div></div>\r | |
82 | <div class="paragraph"><p>The definition of <span class="monospaced">printf</span> is even careful to not print anything until\r | |
83 | it is fully applied. So, examples like the following will work as\r | |
84 | expected.</p></div>\r | |
85 | <div class="listingblock">\r | |
86 | <div class="content monospaced">\r | |
87 | <pre>val f: int -> bool -> unit = printf `"Int="I`" Bool="B`"\n" $ 13\r | |
88 | val () = f true\r | |
89 | val () = f false</pre>\r | |
90 | </div></div>\r | |
91 | <div class="paragraph"><p>It is also easy to define new format specifiers. For example, suppose\r | |
92 | we wanted format specifiers for characters and strings.</p></div>\r | |
93 | <div class="listingblock">\r | |
94 | <div class="content monospaced">\r | |
95 | <pre>val C = fn z => spec Char.toString z\r | |
96 | val S = fn z => spec (fn s => s) z</pre>\r | |
97 | </div></div>\r | |
98 | <div class="paragraph"><p>One can define format specifiers for more complex types, e.g. pairs of\r | |
99 | integers.</p></div>\r | |
100 | <div class="listingblock">\r | |
101 | <div class="content monospaced">\r | |
102 | <pre>val I2 =\r | |
103 | fn z =>\r | |
104 | spec (fn (i, j) =>\r | |
105 | concat ["(", Int.toString i, ", ", Int.toString j, ")"])\r | |
106 | z</pre>\r | |
107 | </div></div>\r | |
108 | <div class="paragraph"><p>Here’s an example use.</p></div>\r | |
109 | <div class="listingblock">\r | |
110 | <div class="content monospaced">\r | |
111 | <pre>val () = printf `"Test "I2`" a string "S`"\n" $ (1, 2) "hello"</pre>\r | |
112 | </div></div>\r | |
113 | </div>\r | |
114 | </div>\r | |
115 | <div class="sect1">\r | |
116 | <h2 id="_printf_via_a_href_fold_fold_a">Printf via <a href="Fold">Fold</a></h2>\r | |
117 | <div class="sectionbody">\r | |
118 | <div class="paragraph"><p><span class="monospaced">printf</span> is best viewed as a special case of variable-argument\r | |
119 | <a href="Fold">Fold</a> that inductively builds a function as it processes its\r | |
120 | arguments. Here is the definition of a <span class="monospaced">Printf</span> structure in terms of\r | |
121 | fold. The structure is equivalent to the above one, except that it\r | |
122 | uses the standard <span class="monospaced">$</span> instead of a specialized one.</p></div>\r | |
123 | <div class="listingblock">\r | |
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>\r | |
125 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
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>\r | |
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">=></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>\r | |
128 | \r | |
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">=></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>\r | |
130 | \r | |
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>\r | |
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">=></span><span class="w"></span>\r | |
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">=></span><span class="w"></span>\r | |
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">=></span><span class="w"></span>\r | |
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">=></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>\r | |
136 | \r | |
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>\r | |
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">=></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>\r | |
139 | \r | |
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">=></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>\r | |
141 | \r | |
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">=></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>\r | |
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">=></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>\r | |
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">=></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>\r | |
145 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
146 | </pre></div></div></div>\r | |
147 | <div class="paragraph"><p>Viewing <span class="monospaced">printf</span> as a fold opens up a number of possibilities. For\r | |
148 | example, one can name parts of format strings using the fold idiom for\r | |
149 | naming sequences of steps.</p></div>\r | |
150 | <div class="listingblock">\r | |
151 | <div class="content monospaced">\r | |
152 | <pre>val IB = fn u => Fold.fold u `"Int="I`" Bool="B\r | |
153 | val () = printf IB`" "IB`"\n" $ 1 true 3 false</pre>\r | |
154 | </div></div>\r | |
155 | <div class="paragraph"><p>One can even parametrize over partial format strings.</p></div>\r | |
156 | <div class="listingblock">\r | |
157 | <div class="content monospaced">\r | |
158 | <pre>fun XB X = fn u => Fold.fold u `"X="X`" Bool="B\r | |
159 | val () = printf (XB I)`" "(XB R)`"\n" $ 1 true 2.0 false</pre>\r | |
160 | </div></div>\r | |
161 | </div>\r | |
162 | </div>\r | |
163 | <div class="sect1">\r | |
164 | <h2 id="_also_see">Also see</h2>\r | |
165 | <div class="sectionbody">\r | |
166 | <div class="ulist"><ul>\r | |
167 | <li>\r | |
168 | <p>\r | |
169 | <a href="PrintfGentle">PrintfGentle</a>\r | |
170 | </p>\r | |
171 | </li>\r | |
172 | <li>\r | |
173 | <p>\r | |
174 | <a href="References#Danvy98"> Functional Unparsing</a>\r | |
175 | </p>\r | |
176 | </li>\r | |
177 | </ul></div>\r | |
178 | </div>\r | |
179 | </div>\r | |
180 | </div>\r | |
181 | <div id="footnotes"><hr></div>\r | |
182 | <div id="footer">\r | |
183 | <div id="footer-text">\r | |
184 | </div>\r | |
185 | <div id="footer-badges">\r | |
186 | </div>\r | |
187 | </div>\r | |
188 | </body>\r | |
189 | </html>\r |