The Project | About Us | Contribute | Donations | License
HOME My courses Completed
◄ Previous
◄

Template Haskell

Next ►
►

Template Haskell is a powerful metaprogramming tool in Haskell that allows you to generate and manipulate Haskell code programmatically. It enables you to write code that writes code, which can be particularly useful for tasks like code generation, optimization, and embedding domain-specific languages.

Key Concepts

  1. Quasi-Quoting: Allows you to embed Haskell code within Haskell code.
  2. Splicing: Inserting generated code into your program.
  3. Reification: Inspecting the structure of Haskell code.
  4. Syntax Trees: Representing Haskell code as data structures.

Setting Up Template Haskell

To use Template Haskell, you need to enable the TemplateHaskell language extension. You can do this by adding the following pragma at the top of your Haskell file:

{-# LANGUAGE TemplateHaskell #-}

Basic Syntax

Quasi-Quoting

Quasi-quoting allows you to write Haskell code as a string and then convert it into an abstract syntax tree (AST). The syntax for quasi-quoting is [| ... |].

Example:

{-# LANGUAGE TemplateHaskell #-}

module Main where

import Language.Haskell.TH

-- Quasi-quoted expression
example :: Q Exp
example = [| 1 + 2 |]

Splicing

Splicing is the process of inserting generated code into your program. The syntax for splicing is $( ... ).

Example:

{-# LANGUAGE TemplateHaskell #-}

module Main where

import Language.Haskell.TH

-- Quasi-quoted expression
example :: Q Exp
example = [| 1 + 2 |]

-- Splicing the expression into the program
main :: IO ()
main = print $(example)

Reification

Reification allows you to inspect the structure of Haskell code. You can use the reify function to get information about a name.

Example:

{-# LANGUAGE TemplateHaskell #-}

module Main where

import Language.Haskell.TH

-- Function to reify
myFunction :: Int -> Int
myFunction x = x + 1

-- Reify the function
main :: IO ()
main = do
  info <- runQ (reify 'myFunction)
  print info

Syntax Trees

Haskell code is represented as syntax trees in Template Haskell. The Q monad is used to construct these trees.

Example:

{-# LANGUAGE TemplateHaskell #-}

module Main where

import Language.Haskell.TH

-- Constructing a syntax tree
example :: Q Exp
example = do
  let x = mkName "x"
  lamE [varP x] (infixE (Just (varE x)) (varE '(+)) (Just (litE (integerL 1))))

-- Splicing the syntax tree into the program
main :: IO ()
main = print $(example)

Practical Example: Generating a Function

Let's create a Template Haskell function that generates a function to add two numbers.

{-# LANGUAGE TemplateHaskell #-}

module Main where

import Language.Haskell.TH

-- Function to generate an addition function
genAddFunction :: Q [Dec]
genAddFunction = do
  let fName = mkName "add"
  let x = mkName "x"
  let y = mkName "y"
  funD fName [clause [varP x, varP y] (normalB [| $(varE x) + $(varE y) |]) []]

-- Splice the generated function into the program
$(genAddFunction)

main :: IO ()
main = print (add 3 5)

Exercises

Exercise 1: Generating a Multiplication Function

Write a Template Haskell function that generates a function to multiply two numbers.

Solution:

{-# LANGUAGE TemplateHaskell #-}

module Main where

import Language.Haskell.TH

-- Function to generate a multiplication function
genMulFunction :: Q [Dec]
genMulFunction = do
  let fName = mkName "mul"
  let x = mkName "x"
  let y = mkName "y"
  funD fName [clause [varP x, varP y] (normalB [| $(varE x) * $(varE y) |]) []]

-- Splice the generated function into the program
$(genMulFunction)

main :: IO ()
main = print (mul 3 5)

Exercise 2: Generating a Function to Compute the Length of a List

Write a Template Haskell function that generates a function to compute the length of a list.

Solution:

{-# LANGUAGE TemplateHaskell #-}

module Main where

import Language.Haskell.TH

-- Function to generate a length function
genLengthFunction :: Q [Dec]
genLengthFunction = do
  let fName = mkName "listLength"
  let xs = mkName "xs"
  funD fName [clause [varP xs] (normalB [| length $(varE xs) |]) []]

-- Splice the generated function into the program
$(genLengthFunction)

main :: IO ()
main = print (listLength [1, 2, 3, 4, 5])

Conclusion

In this section, we explored Template Haskell, a powerful metaprogramming tool in Haskell. We covered the basics of quasi-quoting, splicing, reification, and syntax trees. We also provided practical examples and exercises to help you understand how to generate and manipulate Haskell code programmatically. In the next module, we will delve into concurrency and parallelism in Haskell.

◄ Previous
◄
Next ►
►
The Project | About Us | Contribute | Donations | License
© Copyright 2024. All rights reserved