Primer: Writing efficient code in a lazy language (Hamburg Haskell Meetup 2016-05-24)

RESOURCES.md

Book

• Introduction to Functional Programming by Bird and Wadler (referred as ITFP)
• My solutions to (some) of the exercises in the book implemented in Haskell (PRs and comments are welcome!)

Topics

Models of reduction

• ITFP 6.2.2, 6.2.3
• https://hackhands.com/lazy-evaluation-works-haskell/ (Haskell-specific)

Examples

In ghci:

sqr (6 + 5) -- innermost vs outermost evaluation
fst (6, undefined) -- innermost vs outermost evaluation
let a = [1..] :: [Integer] ; let b = map (+ 1) a ; a !! 6; b !! 11 ; :sprint a ; :sprint b

Exercises

• ITFP 6.2.1, 6.2.2

Efficiency

• ITFP 6.1, 6.2, 6.3
• http://dev.stephendiehl.com/hask/#strictness (Haskell-specific)

Examples

In ghci:

:set -XBangPatterns

fst (1, undefined) -- 1
fst' (!a, !b) = a
fst' (1, undefined) -- 💥

length [undefined, 3, 4, 5] -- 4
-- 🤔 this didn't work in the live demo because I defined it as: g (!x:xs)= length (x:y:xs)
{ length' [] = 0; length' ((!x):xs)= length (x:xs) }
length' [undefined,3,4,5] -- 💥

Exercises

• 6.3.3

Profiling in Haskell

• http://book.realworldhaskell.org/read/profiling-and-optimization.html
• https://downloads.haskell.org/~ghc/8.0.2/docs/html/users_guide/profiling.html
• https://docs.haskellstack.org/en/stable/GUIDE/#debugging
• http://dev.stephendiehl.com/hask/#profiling
• https://github.com/fpco/weigh