From: mthomas <unknown>
Date: Thu, 28 Nov 2002 01:00:44 +0000 (+0000)
Subject: [project @ 2002-11-28 01:00:44 by mthomas]
X-Git-Tag: Approx_11550_changesets_converted~1399
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=2c5ca121806c7ee3d81b728397e6e74ff2909d9d

[project @ 2002-11-28 01:00:44 by mthomas]
Add a getting started example.  Someone with Docbook please check the SGML.
---

diff --git a/ghc/docs/users_guide/glasgow_exts.sgml b/ghc/docs/users_guide/glasgow_exts.sgml
index 7dff721..4d1ee17 100644
--- a/ghc/docs/users_guide/glasgow_exts.sgml
+++ b/ghc/docs/users_guide/glasgow_exts.sgml
@@ -3218,6 +3218,9 @@ Template Meta-programming for Haskell</ulink>", in
 Proc Haskell Workshop 2002.
 </para>
 
+<para> The first example from that paper is set out below as a worked example to help get you started. 
+</para>
+
 <para>
 The documentation here describes the realisation in GHC.  (It's rather sketchy just now;
 Tim Sheard is going to expand it.)
@@ -3301,6 +3304,73 @@ Tim Sheard is going to expand it.)
 </para>
 </sect2>
  
+<sect2>  <title> A Template Haskell Worked Example </title>
+<para>To help you get over the confidence barrier, try out this skeletal worked example.
+  First cut and paste the two modules below into "Main.hs" and "Printf.hs":</para>
+
+<programlisting>
+{- Main.hs -}
+module Main where
+
+-- Import our template "pr"
+import Printf ( pr )
+
+-- The splice operator $ takes the Haskell source code
+-- generated at compile time by "pr" and splices it into
+-- the argument of "putStrLn".
+main = putStrLn ( $(pr "Hello") )
+</programlisting>
+
+<programlisting>
+{- Printf.hs -}
+module Printf where
+
+-- Skeletal printf from the paper.
+-- It needs to be in a separate module to the one where
+-- you intend to use it.
+
+-- Import some Template Haskell syntax
+import Language.Haskell.THSyntax
+
+-- Describe a format string
+data Format = D | S | L String
+
+-- Parse a format string.  This is left largely to you
+-- as we are here interested in building our first ever
+-- Template Haskell program and not in building printf.
+parse :: String -> [Format]
+parse s   = [ L s ]
+
+-- Generate Haskell source code from a parsed representation
+-- of the format string.  This code will be spliced into
+-- the module which calls "pr", at compile time.
+gen :: [Format] -> Expr
+gen [D]   = [| \n -> show n |]
+gen [S]   = [| \s -> s |]
+gen [L s] = string s
+
+-- Here we generate the Haskell code for the splice
+-- from an input format string.
+pr :: String -> Expr
+pr s      = gen (parse s)
+</programlisting>
+
+<para>Now run the compiler (here we are using a "stage three" build of GHC, at a Cygwin prompt on Windows):
+</para>
+<programlisting>
+stage3/ghc/compiler/ghc-inplace --make -fglasgow-exts -package haskell-src main.hs -o main.exe
+</programlisting>
+
+<para>Run "main.exe" and here is your output:
+</para>
+
+<programlisting>
+$ ./main
+Hello
+</programlisting>
+
+</sect2>
+ 
 </sect1>
 
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