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<HTML>
<HEAD>
<TITLE>Haskell 98: ERRATA</TITLE>
</HEAD>
<BODY>
<div align=center>
<img src="h98.gif" alt="Haskell 98">
</div>
<h1>Errata in the
<a href="http://haskell.cs.yale.edu/definition/">Haskell 98 Language Report</a></h1>
<ul>
<p><li> <strong>Title page</strong>. Add the paragraph:
<p>
"Copyright (c) Simon Peyton Jones and John Hughes.
<p>
The authors intend this Report to belong to the entire Haskell
community, and so we grant permission to copy and
distribute it for any purpose, provided that it is
reproduced in its entireity, including this Notice. Modified
versions of this Report may also be copied and distributed for any purpose,
provided that the modified version is clearly presented as such,
and that it does not claim to be a definition of the language Haskell 98."
<p><li> <strong>Page 10, Section 2.7, Layout.</strong>
In the middle of the third paragraph, just before the sentence beginning
"A close brace is also inserted...", add the following sentence:
<p>
"If the indentation of the non-brace lexeme immediately
following a <tt>where</tt>, <tt>let</tt>,
<tt>do</tt> or <tt>of</tt> is less than or equal to the <em>current</em> indentation level,
then instead of starting a layout, an empty list "<tt>{}</tt>" is inserted, and
layout processing occurs for the current level (i.e. insert a semicolon or close brace)."
<p><li> <strong>Page 11, Figures 1 and 2</strong>. In both figures the signature for <tt>size</tt>
should be
<pre>
size :: Stack a -> Int
</pre>
<p><li> <strong>Page 14, Section 3.1, Errors.</strong> In the first sentence of
the section, after "indistinguishable" add "by a Haskell program".
<p><li> <strong>Page 20, Section 3.10, Arithmetic sequences.</strong>
In the second paragraph, in the sentence "For the type <tt>Integer</tt>,
arithmetic sequences have the following meaning...", replace "<tt>Integer</tt>"
with "<tt>Int</tt>".
<p><li> <strong>Page 23, Section 3.14, Do Expressions;
and Page 128, Appendix B.4, Context Free Syntax.</strong>
<ul>
<li>
Change the production for <em>stmts</em> to read:
<pre>
stmts -> stmt1 ... stmtn exp [;] (n>=0)
</pre>
<li> Add "<tt>;</tt>" to the end of each of the four productions for <em>stmt</em>.
</ul>
That is, every list of statements must end in an expression, optionally
followed by a semicolon.
<p><li> <strong>Page 40, Section 4.2.1, Algebraic Datatype Declarations.</strong>
In the bottom paragraph on the page, before "The context in the data declaration
has no other effect whatsoever" add the following:
<p>
"Pattern matching against <tt>ConsSet</tt> also gives rise to an <tt>Eq a</tt>
constraint. For example:
<pre>
f (ConsSet a s) = a
</pre>
the function <tt>f</tt> has inferred type <tt>Eq a => Set a -> a</tt>."
<p><li> <strong>Page 41, Section 4.2.1, subsection Labelled Fields.</strong>
At the end of the subsection, add a new paragraph:
<p>
"The pattern "<tt>F {}</tt>" matches any value built with constructor <tt>F</tt>,
<em>whether or not <tt>F</tt> was declared with record syntax</em>."
<p><li> <strong>Page 49, Section 4.3.4, Ambiguous Types...</strong>
In the third paragraph from the end of Section 4.3.4, replace "...an ambiguous
type variable is defaultable if at least one of its classes is a numeric class..."
by "...an ambiguous type variable, v, is defaultable if v appears only in constraints
of the form (C v), where C is a class, and if at least one of its classes is
a numeric class...".
<p>
The new phrase is "if v appears only in constraints of the
form (C v) where C is a class". Without this condition the rest of the
sentence does not make sense.
<p><li> <strong>Page 57, Section 4.5.3, Context reduction errors.</strong>
The example
<pre>
f :: (Monad m, Eq (m a)) => a -> m a -> Bool
f x y = x == return y
</pre>
is wrong; it should read
<pre>
f x y = return x == y
</pre>
<p><li> <strong>Page 66, Section 5.3, Import Declarations, numbered item 2.</strong>
Start a new paragraph before the sentence "The hiding clause only applies to unqualified
names...".
<p><li><strong>Page 67, Section 5.3.2, Local aliases</strong>.
The the last example in the section should read:
<pre>
import Foo as A(f)
</pre>
<p><li> <strong>Page 69, Section 5.5.2, Name clashes.</strong>
At the very end of the section, add the following clarification:
<p>
"The name occurring in a type signature or fixity declarations is always
unqualified, and unambiguously refers to another declaration in the
same declaration list (except that the fixity declaration for a class method
can occur at top level --- Section 4.4.2). For example, the following
module is legal:
<pre>
module F where
sin :: Float -> Float
sin x = (x::Float)
f x = Prelude.sin (F.sin x)
</pre>
The local declaration for <tt>sin</tt> is legal, even though
the <tt>Prelude</tt> function <tt>sin</tt> is implicitly in scope.
The references to
<tt>Prelude.sin</tt> and <tt>F.sin</tt> must both be qualified to make
it unambigous which <tt>sin</tt> is meant.
However, the unqualified name "<tt>sin</tt>" in the type
signature in the first line of <tt>F</tt> unambiguously refers to the
local declaration for <tt>sin</tt>."
<p><li> <strong>Page 71, Section 5.6.2, Shadowing Prelude Names.</strong>
Replace the example at the beginning of the section, and the entire
following paragraph, with the following
<pre>
module A( null, nonNull ) where
import Prelude hiding( null )
null, nonNull :: Int -> Bool
null x = x == 0
nonNull x = not (null x)
</pre>
Module <tt>A</tt> redefines <tt>null</tt>, and contains an unqualified reference
to <tt>null</tt> on the right hand side of <tt>nonNull</tt>.
The latter would be ambiguous without the "<tt>hiding(null)</tt>" on the
"<tt>import Prelude</tt>" statement.
Every module that imports <tt>A</tt> unqualified, and then makes an unqualified
reference to <tt>null</tt> must also resolve the ambiguous use of <tt>null</tt>
just as <tt>A</tt> does. Thus there is little danger of accidentally shadowing
Prelude names.
<p><li> <strong>Page 76, Section 6.3, Standard Haskell classes.</strong>
There is a formatting error in the example. The sentence beginning "This declaration gives default
method declarations..." should start on a new line.
<p><li><strong>Page 80, Section 6.3.6, Class Monad.</strong>
Right at the bottom of the page, after "However, for IO, the fail
method invokes error." add the clarifying sentence "To raise
a user exception in the IO monad, use <tt>ioError</tt> (see
Section 7.3)."
<p><li> <strong>Page 84, Section 6.4, Fig 7; and bottom of page 86, Section 6.4.6.</strong>
<pre>
fromRealFrac :: (RealFrac a, Fractional b) => a -> b
</pre>
should be replaced by
<pre>
realToFrac :: (Real a, Fractional b) => a -> b
</pre>
<p><li><strong>Page 90, Section 7.3, Exception handling in the IO monad.</strong>
After the first paragraph give the following type signature:
<pre>
userError :: String -> IOError
</pre>
Also add an index entry for <tt>userError</tt> on this page.
(These changes are purely presentational.)
<p><li><strong>Page 94, Appendix A, Standard Prelude, class <tt>Enum</tt>.</strong>
After the default method for <tt>enumFromTo</tt> add
<pre>
enumFromThen x y = map toEnum [fromEnum x, fromEnum y ..]
</pre>
<p><li><strong>Page 102, Appendix A, comments immediately before
<tt>instance Enum Float</tt>.</strong>
Replace "<tt>1.0</tt>" by "<tt>0.95</tt>".
<p><li><strong>Page 105, Appendix A.1, line 11.</strong>
In the module header for <tt>PreludeList</tt> replace "<tt>Sum</tt>" by "<tt>sum</tt>".
<p><li><strong>Page 106, definition of <tt>(!!)</tt>.</strong>
Replace the definition of <tt>(!!)</tt> with the following:
<pre>
-- List index (subscript) operator, 0-origin
(!!) :: [a] -> Int -> a
xs !! n | n < 0 = error "Prelude.!!: negative index"
[] !! _ = error "Prelude.!!: index too large"
(x:_) !! 0 = x
(_:xs) !! n = xs !! (n-1)
</pre>
(The original version had the property that <tt>([] !! -100)</tt>
reported "index too large".)
<p><li> <strong>Page 141, Bibliograpy</strong>.
The library report citation [8] is out of date. It should read:
"Simon Peyton Jones and John Hughes (editors), Standard Libraries for the
Haskell 98 Programming Language, 1 February 1999".
</ul>
<hr>
<h1>Errata in the
<a href="http://haskell.cs.yale.edu/definition/">Haskell 98 Library Report</a></h1>
<ul>
<p><li> <strong>Title page</strong>. Add the paragraph:
<p>
"Copyright (c) Simon Peyton Jones and John Hughes.
<p>
The authors intend this Report to belong to the entire Haskell
community, and so we grant permission to copy and
distribute it for any purpose, provided that it is
reproduced in its entireity, including this Notice. Modified
versions of this Report may also be copied and distributed for any purpose,
provided that the modified version is clearly presented as such,
and that it does not claim to be a definition of the Haskell 98 libraries."
<p><li> <strong>Page 17, Section 5, first text line</strong>. Replace "continuous" by "contiguous".
<p><li> <strong>Page 29, Section 7.2, second bullet</strong>.
Introduce a new bullet just before "<tt>union</tt> is list union".
<p><li><strong>Page 64, Section 12, Directory functions.</strong>
(More an editor's blunder than a typo.)
<ul>
<li>
Replace the abstract definition of <tt>Permissions</tt>, and the four instances
of <tt>Permissions</tt> with
<pre>
data Permissions
= Permissions {
readable, writable, executable, searchable :: Bool
}
deriving ( Eq, Ord, Read, Show )
</pre>
<li> Remove <tt>readable, writable, executable, searchable</tt> both from
the module header, and from the module body.
<li> Change <tt>Permissions</tt> to <tt>Permissions(readable, writable, executable, searchable)</tt>
in the module header.
</ul>
</ul>
<hr>
<address>Simon Peyton Jones, simonpj@microsoft.com</address>
</body>
</html>
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