51 lines
1.2 KiB
Elm
51 lines
1.2 KiB
Elm
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module Lazy where
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{-| Library for efficient Lazy evaluation and Infinite Streams.
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-}
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import Basics ((-))
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import Signal ((<~), foldp, Signal)
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import Native.Lazy
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data Lazy a = L { run : () -> a }
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{-| Memoize a thunk so it is evaluated at most once -}
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thunk : (() -> a) -> Lazy a
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thunk t = L { run = (Native.Lazy.thunk t) }
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run : Lazy a -> a
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run l = case l of L r -> r.run ()
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data Stream a = S { hd : a, tl : Lazy (Stream a) }
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hd : Stream a -> a
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hd (S r) = r.hd
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tl : Stream a -> Stream a
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tl (S r) = run (r.tl)
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repeat : a -> Stream a
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repeat x = S { hd = x, tl = thunk (\() -> repeat x) }
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cycle : a -> [a] -> Stream a
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cycle x xs = let cycle' ys = case ys of
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[] -> S { hd = x, tl = thunk (\() -> cycle' xs) }
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(y :: ys) -> S { hd = y, tl = thunk (\() -> cycle' ys) }
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in cycle' []
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zipWith : (a -> b -> c) -> Stream a -> Stream b -> Stream c
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zipWith f xs ys = S { hd = f (hd xs) (hd ys)
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, tl = thunk (\() -> zipWith f (tl xs) (tl ys)) }
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observeWhen : Stream a -> Signal b -> Signal a
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observeWhen str sig = let tls = foldp (\_ -> tl) str sig in
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hd <~ tls
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take : Int -> Stream a -> [a]
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take n xs = case n of
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0 -> []
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n -> hd xs :: take (n - 1) (tl xs)
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