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Programmers coming from C or Java often ask if <a href="StandardML">Standard ML</a> has a <tt>printf</tt> function. It does not. However, it is possible to implement your own version with only a few lines of code. <p>
Here is a definition for <tt>printf</tt> and <tt>fprintf</tt>, along with format specifiers for booleans, integers, and reals.
</p>
<pre class=code>
<B><FONT COLOR="#0000FF">structure</FONT></B> Printf =
<B><FONT COLOR="#0000FF">struct</FONT></B>
<B><FONT COLOR="#A020F0">fun</FONT></B> $ (_, f) = f (<B><FONT COLOR="#A020F0">fn</FONT></B> p => p ()) ignore
<B><FONT COLOR="#A020F0">fun</FONT></B> fprintf out f = f (out, id)
<B><FONT COLOR="#A020F0">val</FONT></B> printf = <B><FONT COLOR="#A020F0">fn</FONT></B> z => fprintf TextIO.stdOut z
<B><FONT COLOR="#A020F0">fun</FONT></B> one ((out, f), make) g =
g (out, <B><FONT COLOR="#A020F0">fn</FONT></B> r =>
f (<B><FONT COLOR="#A020F0">fn</FONT></B> p =>
make (<B><FONT COLOR="#A020F0">fn</FONT></B> s =>
r (<B><FONT COLOR="#A020F0">fn</FONT></B> () => (p (); TextIO.output (out, s))))))
<B><FONT COLOR="#A020F0">fun</FONT></B> ` x s = one (x, <B><FONT COLOR="#A020F0">fn</FONT></B> f => f s)
<B><FONT COLOR="#A020F0">fun</FONT></B> spec to x = one (x, <B><FONT COLOR="#A020F0">fn</FONT></B> f => f o to)
<B><FONT COLOR="#A020F0">val</FONT></B> B = <B><FONT COLOR="#A020F0">fn</FONT></B> z => spec Bool.toString z
<B><FONT COLOR="#A020F0">val</FONT></B> I = <B><FONT COLOR="#A020F0">fn</FONT></B> z => spec Int.toString z
<B><FONT COLOR="#A020F0">val</FONT></B> R = <B><FONT COLOR="#A020F0">fn</FONT></B> z => spec Real.toString z
<B><FONT COLOR="#0000FF">end</FONT></B>
</PRE>
<p>
</p>
<p>
Here's an example use.
</p>
<pre class=code>
<B><FONT COLOR="#A020F0">val</FONT></B> () = printf `<B><FONT COLOR="#BC8F8F">"Int="</FONT></B>I`<B><FONT COLOR="#BC8F8F">" Bool="</FONT></B>B`<B><FONT COLOR="#BC8F8F">" Real="</FONT></B>R`<B><FONT COLOR="#BC8F8F">"\n"</FONT></B> $ <B><FONT COLOR="#5F9EA0">1</FONT></B> false <B><FONT COLOR="#5F9EA0">2.0</FONT></B>
</PRE>
<p>
</p>
<p>
This prints the following.
<pre>Int=1 Bool=false Real=2.0
</pre>
</p>
<p>
In general, a use of <tt>printf</tt> looks like
</p>
<pre>printf <spec1> ... <specn> $ <arg1> ... <argm>
</pre><p>
where each <tt><speci></tt> is either a specifier like <tt>B</tt>, <tt>I</tt>, or <tt>R</tt>, or is an inline string, like <tt>`"foo"</tt>. A backtick (<tt>`</tt>) must precede each inline string. Each <tt><argi></tt> must be of the appropriate type for the corresponding specifier.
</p>
<p>
SML <tt>printf</tt> is more powerful than its C counterpart in a number of ways. In particular, the function produced by <tt>printf</tt> is a perfectly ordinary SML function, and can be passed around, used multiple times, etc. For example:
</p>
<pre class=code>
<B><FONT COLOR="#A020F0">val</FONT></B> f: int -> bool -> unit = printf `<B><FONT COLOR="#BC8F8F">"Int="</FONT></B>I`<B><FONT COLOR="#BC8F8F">" Bool="</FONT></B>B`<B><FONT COLOR="#BC8F8F">"\n"</FONT></B> $
<B><FONT COLOR="#A020F0">val</FONT></B> () = f <B><FONT COLOR="#5F9EA0">1</FONT></B> true
<B><FONT COLOR="#A020F0">val</FONT></B> () = f <B><FONT COLOR="#5F9EA0">2</FONT></B> false
</PRE>
<p>
</p>
<p>
The definition of <tt>printf</tt> is even careful to not print anything until it is fully applied. So, examples like the following will work as expected.
</p>
<pre>val f: int -> bool -> unit = printf `"Int="I`" Bool="B`"\n" $ 13
val () = f true
val () = f false
</pre><p>
It is also easy to define new format specifiers. For example, suppose we wanted format specifiers for characters and strings.
</p>
<pre>val C = fn z => spec Char.toString z
val S = fn z => spec (fn s => s) z
</pre><p>
One can define format specifiers for more complex types, e.g. pairs of integers.
</p>
<pre>val I2 =
fn z =>
spec (fn (i, j) =>
concat ["(", Int.toString i, ", ", Int.toString j, ")"])
z
</pre><p>
Here's an example use.
</p>
<pre>val () = printf `"Test "I2`" a string "S`"\n" $ (1, 2) "hello"
</pre><h2 id="head-61fc0a9cf6f322aec07bfaef46205ce788f00071">Printf via fold</h2>
<p>
Printf 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 <tt>Printf</tt> structure in terms of fold. The structure is equivalent to the above one, except that it uses the standard <tt>$</tt> instead of a specialized one.
</p>
<pre class=code>
<B><FONT COLOR="#0000FF">structure</FONT></B> Printf =
<B><FONT COLOR="#0000FF">struct</FONT></B>
<B><FONT COLOR="#A020F0">fun</FONT></B> fprintf out =
Fold.fold ((out, id), <B><FONT COLOR="#A020F0">fn</FONT></B> (_, f) => f (<B><FONT COLOR="#A020F0">fn</FONT></B> p => p ()) ignore)
<B><FONT COLOR="#A020F0">val</FONT></B> printf = <B><FONT COLOR="#A020F0">fn</FONT></B> z => fprintf TextIO.stdOut z
<B><FONT COLOR="#A020F0">fun</FONT></B> one ((out, f), make) =
(out, <B><FONT COLOR="#A020F0">fn</FONT></B> r =>
f (<B><FONT COLOR="#A020F0">fn</FONT></B> p =>
make (<B><FONT COLOR="#A020F0">fn</FONT></B> s =>
r (<B><FONT COLOR="#A020F0">fn</FONT></B> () => (p (); TextIO.output (out, s))))))
<B><FONT COLOR="#A020F0">val</FONT></B> ` =
<B><FONT COLOR="#A020F0">fn</FONT></B> z => Fold.step1 (<B><FONT COLOR="#A020F0">fn</FONT></B> (s, x) => one (x, <B><FONT COLOR="#A020F0">fn</FONT></B> f => f s)) z
<B><FONT COLOR="#A020F0">fun</FONT></B> spec to = Fold.step0 (<B><FONT COLOR="#A020F0">fn</FONT></B> x => one (x, <B><FONT COLOR="#A020F0">fn</FONT></B> f => f o to))
<B><FONT COLOR="#A020F0">val</FONT></B> B = <B><FONT COLOR="#A020F0">fn</FONT></B> z => spec Bool.toString z
<B><FONT COLOR="#A020F0">val</FONT></B> I = <B><FONT COLOR="#A020F0">fn</FONT></B> z => spec Int.toString z
<B><FONT COLOR="#A020F0">val</FONT></B> R = <B><FONT COLOR="#A020F0">fn</FONT></B> z => spec Real.toString z
<B><FONT COLOR="#0000FF">end</FONT></B>
</PRE>
<p>
</p>
<p>
Viewing <tt>printf</tt> 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>
<pre>val IB = fn u => Fold.fold u `"Int="I`" Bool="B
val () = printf IB`" "IB`"\n" $ 1 true 3 false
</pre><p>
One can even parametrize over partial format strings.
</p>
<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><h2 id="head-a4bc8bf5caf54b18cea9f58e83dd4acb488deb17">Also see</h2>
<ul>
<li>
<p>
<a href="PrintfGentle">PrintfGentle</a>
</p>
</li>
<li>
<p>
<a href = "References#Danvy98"> Functional Unparsing</a>
</p>
</li>
</ul>
</div>
<p>
<hr>
Last edited on 2007-08-15 22:07:07 by <span title="fenrir.uchicago.edu"><a href="MatthewFluet">MatthewFluet</a></span>.
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