Question

I have a trivial Template Haskell program that prints the name of the current module (Main, here):

{-# LANGUAGE TemplateHaskell #-}
module Main
( main
) where

import Language.Haskell.TH
import Language.Haskell.TH.Syntax

modName ∷ String
modName = $(fmap loc_module qLocation »= λmod → return (LitE (StringL mod) ))

main ∷ IO ()
main = putStrLn modName

When I compile this, I get the following Loading messages from ghc:

tsuraan@localhost ~/test/modname $ ghc --make Main
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package array-0.4.0.0 ... linking ... done.
Loading package deepseq-1.3.0.0 ... linking ... done.
Loading package containers-0.4.2.1 ... linking ... done.
Loading package pretty-1.1.1.0 ... linking ... done.
Loading package template-haskell ... linking ... done.
Linking Main ...

Why does ghc load all these packages when Template Haskell is enabled? Whenever I build a program that uses Template Haskell, especially one that is built against a lot of packages, my compile warnings are overwhelmed with these superfluous “Loading” messages. It would be nice if I could stop the messages from being printed, or stop the (unnecessary?) module loading from happening at all.

Answer 1

Template Haskell runs at compile time, via a bytecode interpreter (GHCi). Any package dependencies that you have — at compile time — will be loaded dynamically into GHC — at compile time, so that you can execute your splices.

One of your dependencies is the Template Haskell library itself, which in turn depends on most of the core things.