Programming in Haskell

Transcription

1 January 25, 2013

2 Writing Fast Haskell Laziness is both blessing and curse minimum = head. sort is O(n) for carefully-defined sort Memory usage seq, $! and bang patterns

3 Memoization Any thunk is only ever evaluted once Store thunks in a map indexed by the argument! Example: memofibs Data.MemoTrie and Data.MemoCombinators

4 Weak head normal form Strict: if f undefined = undefined

5 Weak head normal form Strict: if f undefined = undefined weak head normal form

6 Weak head normal form Strict: if f undefined = undefined weak head normal form For constructors, is different from undefined; e.g. [1,2,undefined], Just "whnf"

7 Weak head normal form Strict: if f undefined = undefined weak head normal form For constructors, is different from undefined; e.g. [1,2,undefined], Just "whnf" For functions, is not fully applied: const 2 3 is fully applied, const 2 is not

8 Weak head normal form Strict: if f undefined = undefined weak head normal form For constructors, is different from undefined; e.g. [1,2,undefined], Just "whnf" For functions, is not fully applied: const 2 3 is fully applied, const 2 is not Lambda expressions are always in WHNF

9 Weak head normal form Strict: if f undefined = undefined weak head normal form For constructors, is different from undefined; e.g. [1,2,undefined], Just "whnf" For functions, is not fully applied: const 2 3 is fully applied, const 2 is not Lambda expressions are always in WHNF Just undefined seq 2, [1..] seq 2 both evaluate to 2 and perform no extra forcing

10 Weak head normal form Strict: if f undefined = undefined weak head normal form For constructors, is different from undefined; e.g. [1,2,undefined], Just "whnf" For functions, is not fully applied: const 2 3 is fully applied, const 2 is not Lambda expressions are always in WHNF Just undefined seq 2, [1..] seq 2 both evaluate to 2 and perform no extra forcing What about 10 ^ 100 ^ 4 seq 2?

11 Weak head normal form Strict: if f undefined = undefined weak head normal form For constructors, is different from undefined; e.g. [1,2,undefined], Just "whnf" For functions, is not fully applied: const 2 3 is fully applied, const 2 is not Lambda expressions are always in WHNF Just undefined seq 2, [1..] seq 2 both evaluate to 2 and perform no extra forcing What about 10 ^ 100 ^ 4 seq 2? Both instances of (^) are fully applied, so haskell will compute

12 Styling and Profiling First: compile your programs like ghc --make -rtsopts -prof program.hs program +RTS -s will output basic profiling information to stderr program +RTS -p will output time profiling information to program.prof program +RTS -i hc

13 Seq-ing strictnes How to use seq properly: let shared = expensive x in shared seq f shared

14 Seq-ing strictnes How to use seq properly: let shared = expensive x in shared seq f shared shared is reduced to WHNF

15 Seq-ing strictnes How to use seq properly: let shared = expensive x in shared seq f shared shared is reduced to WHNF How not to use seq: expensive x seq f (expensive x)

16 Other speedups Bang-patterns: let!x = in x + 6 forces x Compiling with -O2 Don t just throw seq everywhere!

17 Testing with Hspec Unit tests Autogenerated tests using QuickCheck!

18 Other libraries lens for first-class data access netwire for functional reactive programming OpenGL bindings