Compile-time calculation: Difference between revisions

With Template Haskell, it is quite easy to do compile time embedding. The functions used at compile-time need to be already compiled. Therefore, you generally need two modules.

<lang haskell>module Factorial where

import Language.Haskell.TH.Syntax

factQ :: Integer -> Q Exp

factQ = lift . fact</lang>

<lang haskell>{-# LANGUAGE TemplateHaskell #-}

import Factorial

main = print $(factQ 10)</lang>

Note: Doing <code>$([|fact 10|])</code> is the same than doing <code>fact 10</code>. <code>[|something|]</code> returns the abstract syntax tree of <code>something</code>. Thus <code>[|fact 10|]</code> returns the AST of the call to the <code>fact</code> function with 10 as argument. <code>$(something)</code> waits for an AST from a call to <code>something</code>.