[[project @ 2002-11-28 01:00:44 by mthomas]
mthomas**20021128010044
Add a getting started example. Someone with Docbook please check the SGML.
] {
hunk ./ghc/docs/users_guide/glasgow_exts.sgml 3221
+ The first example from that paper is set out below as a worked example to help get you started.
+
+
hunk ./ghc/docs/users_guide/glasgow_exts.sgml 3305
+
+
+ A Template Haskell Worked Example
+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":
+
+
+{- 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 ( $(pr "Hello") )
+
+
+
+{- Printf.hs -}
+module Printf where
+
+-- Skeletal printf from the paper.
+-- It needs to be in a separate module to the one where
+-- you intend to use it.
+
+-- Import some Template Haskell syntax
+import Language.Haskell.THSyntax
+
+-- Describe a format string
+data Format = D | S | L String
+
+-- Parse a format string. This is left largely to you
+-- as we are here interested in building our first ever
+-- Template Haskell program and not in building printf.
+parse :: String -> [Format]
+parse s = [ L s ]
+
+-- Generate Haskell source code from a parsed representation
+-- of the format string. This code will be spliced into
+-- the module which calls "pr", at compile time.
+gen :: [Format] -> Expr
+gen [D] = [| \n -> show n |]
+gen [S] = [| \s -> s |]
+gen [L s] = string s
+
+-- Here we generate the Haskell code for the splice
+-- from an input format string.
+pr :: String -> Expr
+pr s = gen (parse s)
+
+
+Now run the compiler (here we are using a "stage three" build of GHC, at a Cygwin prompt on Windows):
+
+
+stage3/ghc/compiler/ghc-inplace --make -fglasgow-exts -package haskell-src main.hs -o main.exe
+
+
+Run "main.exe" and here is your output:
+
+
+
+$ ./main
+Hello
+
+
}