1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta name="robots" content="index,nofollow">
<title>DefineTypeBeforeUse - MLton Standard ML Compiler (SML Compiler)</title>
<link rel="stylesheet" type="text/css" charset="iso-8859-1" media="all" href="common.css">
<link rel="stylesheet" type="text/css" charset="iso-8859-1" media="screen" href="screen.css">
<link rel="stylesheet" type="text/css" charset="iso-8859-1" media="print" href="print.css">
<link rel="Start" href="Home">
</head>
<body lang="en" dir="ltr">
<script src="http://www.google-analytics.com/urchin.js" type="text/javascript">
</script>
<script type="text/javascript">
_uacct = "UA-833377-1";
urchinTracker();
</script>
<table bgcolor = lightblue cellspacing = 0 style = "border: 0px;" width = 100%>
<tr>
<td style = "
border: 0px;
color: darkblue;
font-size: 150%;
text-align: left;">
<a class = mltona href="Home">MLton MLTONWIKIVERSION</a>
<td style = "
border: 0px;
font-size: 150%;
text-align: center;
width: 50%;">
DefineTypeBeforeUse
<td style = "
border: 0px;
text-align: right;">
<table cellspacing = 0 style = "border: 0px">
<tr style = "vertical-align: middle;">
</table>
<tr style = "background-color: white;">
<td colspan = 3
style = "
border: 0px;
font-size:70%;
text-align: right;">
<a href = "Home">Home</a>
<a href = "TitleIndex">Index</a>
</table>
<div id="content" lang="en" dir="ltr">
<a href="StandardML">Standard ML</a> requires types to be defined before they are used. Because of type inference, the use of a type can be implicit; hence, this requirement is more subtle than it might appear. For example, the following program is not type correct, because the type of <tt>r</tt> is <tt>t option ref</tt>, but <tt>t</tt> is defined after <tt>r</tt>.
<pre class=code>
<B><FONT COLOR="#A020F0">val</FONT></B> r = ref NONE
<B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">A</FONT> </FONT></B>|<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">B</FONT>
</FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> () = r := SOME A
</PRE>
<p>
</p>
<p>
MLton reports the following error, indicating that the type defined on line 2 is used on line 1.
</p>
<pre>Error: z.sml 1.1.
Type escapes the scope of its definition at z.sml 2.10.
type: t
in: val r = ref NONE
</pre><p>
While the above example is benign, the following example shows how to cast an integer to a function by (implicitly) using a type before it is defined. In the example, the ref cell <tt>r</tt> is of type <tt>t option ref</tt>, where <tt>t</tt> is defined <em>after</em> <tt>r</tt>, as a parameter to functor <tt>F</tt>. This example causes <a href="PolyML">PolyML</a> 4.1.3 to seg fault.
<pre class=code>
<B><FONT COLOR="#A020F0">val</FONT></B> r = ref NONE
<B><FONT COLOR="#0000FF">functor</FONT></B> F (<B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> t
</FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> x: t) =
<B><FONT COLOR="#0000FF">struct</FONT></B>
<B><FONT COLOR="#A020F0">val</FONT></B> () = r := SOME x
<B><FONT COLOR="#A020F0">fun</FONT></B> get () = valOf (!r)
<B><FONT COLOR="#0000FF">end</FONT></B>
<B><FONT COLOR="#0000FF">structure</FONT></B> S1 = F (<B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> unit -> unit
</FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> x = <B><FONT COLOR="#A020F0">fn</FONT></B> () => ())
<B><FONT COLOR="#0000FF">structure</FONT></B> S2 = F (<B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> int
</FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> x = <B><FONT COLOR="#5F9EA0">13</FONT></B>)
<B><FONT COLOR="#A020F0">val</FONT></B> () = S1.get () ()
</PRE>
</p>
<p>
MLton reports the following error.
</p>
<pre>Warning: z.sml 1.1.
Unable to locally determine type of variable: r.
type: ??? option ref
in: val r = ref NONE
Error: z.sml 1.1.
Type escapes the scope of its definition at z.sml 2.17.
type: t
in: val r = ref NONE
</pre></div>
<p>
<hr>
Last edited on 2005-12-01 03:38:39 by <span title="ppp-71-139-183-221.dsl.snfc21.pacbell.net"><a href="StephenWeeks">StephenWeeks</a></span>.
</body></html>
|
