Commit 14ce3bb4 authored by simonpj's avatar simonpj
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[project @ 2001-06-11 14:15:32 by simonpj]

document data types with no constructors
parent 1c4e5ade
......@@ -95,6 +95,13 @@ Executive summary of our extensions:
<term>Data types with no constructors</term>
<para>See <xref LinkEnd="nullary types">.</para>
<term>Parallel list comprehensions</term>
......@@ -415,6 +422,24 @@ The libraries documentatation gives more details on all these
<sect1 id="nullary types">
<title>Data types with no constructors</title>
<para>With the <option>-fglasgow-exts</options> flag, GHC lets you declare
a data type with no constructors. For example:</para>
data S -- S :: *
data T a -- T :: * -> *
<para>Syntactically, the declaration lacks the "= constrs" part. The
type can be parameterised, but only over ordinary types, of kind *; since
Haskell does not have kind signatures, you cannot parameterise over higher-kinded
<para>Such data types have only one value, namely bottom.
Nevertheless, they can be useful when defining "phantom types".</para>
<sect1 id="pattern-guards">
<title>Pattern guards</title>
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