Commit 8628e3c4 authored by simonpj's avatar simonpj
Browse files

[project @ 2004-02-03 16:46:33 by simonpj]

Add comments about Template Haskell
parent 29834dd5
......@@ -1598,7 +1598,7 @@ declarations
</programlisting>
"overlap" if <literal>type1</literal> and <literal>type2</literal> unify.
</para>
<para>
However, if you give the command line option
<option>-fallow-overlapping-instances</option><indexterm><primary>-fallow-overlapping-instances
......@@ -3184,6 +3184,12 @@ the background to
the main technical innovations is discussed in "<ulink
url="http://research.microsoft.com/~simonpj/papers/meta-haskell">
Template Meta-programming for Haskell</ulink>" (Proc Haskell Workshop 2002).
The details of the Template Haskell design are still in flux. Make sure you
consult the <ulink url="http://www.haskell.org/ghc/docs/latest/html/libraries/index.html">online library reference material</ulink>
(search for the type ExpQ).
[Temporary: many changes to the original design are described in
<ulink url="http://research.microsoft.com/~simonpj/tmp/notes2.ps">"http://research.microsoft.com/~simonpj/tmp/notes2.ps"</ulink>.
Not all of these changes are in GHC 6.2.]
</para>
<para> The first example from that paper is set out below as a worked example to help get you started.
......@@ -3289,6 +3295,7 @@ Tim Sheard is going to expand it.)
First cut and paste the two modules below into "Main.hs" and "Printf.hs":</para>
<programlisting>
{- Main.hs -}
module Main where
......@@ -3299,9 +3306,8 @@ import Printf ( pr )
-- 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
......@@ -3324,14 +3330,14 @@ 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 :: [Format] -> ExpQ
gen [D] = [| \n -> show n |]
gen [S] = [| \s -> s |]
gen [L s] = string s
gen [L s] = stringE s
-- Here we generate the Haskell code for the splice
-- from an input format string.
pr :: String -> Expr
pr :: String -> ExpQ
pr s = gen (parse s)
</programlisting>
......
Supports Markdown
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment