Commit 2c5ca121 authored by mthomas's avatar mthomas
Browse files

[project @ 2002-11-28 01:00:44 by mthomas]

Add a getting started example.  Someone with Docbook please check the SGML.
parent a551f1a1
......@@ -3218,6 +3218,9 @@ Template Meta-programming for Haskell</ulink>", in
Proc Haskell Workshop 2002.
<para> The first example from that paper is set out below as a worked example to help get you started.
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.)
<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>
{- 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") )
{- 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)
<para>Now run the compiler (here we are using a "stage three" build of GHC, at a Cygwin prompt on Windows):
stage3/ghc/compiler/ghc-inplace --make -fglasgow-exts -package haskell-src main.hs -o main.exe
<para>Run "main.exe" and here is your output:
$ ./main
<!-- ==================== ASSERTIONS ================= -->
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