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copied!<p>Unforunately, you can't currently create a context that requires that an unboxed array be available for a specific type. Quantified Constraints aren't allowed. However, you can still accomplish what you're trying to do, (with the added advantage of having type-specific code versions.) For Longer functions, you could try to split out common expressions so that the repeated code is as small as possible.</p> <pre><code>{-# LANGUAGE FlexibleContexts, ScopedTypeVariables #-} module AccumST where import Control.Monad import Control.Monad.ST import Data.Array.Unboxed import Data.Array.ST import Data.Array.IArray -- General one valid for all instances of Num. -- accumST :: forall a. (IArray UArray a, Num a) => [a] -> Int -> a accumST :: forall a. (IArray UArray a, Num a) => [a] -> Int -> a accumST vals = (!) . runSTArray $ do arr <- newArray (0, length vals) 0 :: (Num a) => ST s (STArray s Int a) forM_ (zip vals [1 .. length vals]) $ \(val, i) -> readArray arr (i - 1) >>= writeArray arr i . (+ val) return arr accumSTFloat vals = (!) . runSTUArray $ do arr <- newArray (0, length vals) 0 :: ST s (STUArray s Int Float) forM_ (zip vals [1 .. length vals]) $ \(val, i) -> readArray arr (i - 1) >>= writeArray arr i . (+ val) return arr accumSTDouble vals = (!) . runSTUArray $ do arr <- newArray (0, length vals) 0 :: ST s (STUArray s Int Double) forM_ (zip vals [1 .. length vals]) $ \(val, i) -> readArray arr (i - 1) >>= writeArray arr i . (+ val) return arr {-# RULES "accumST/Float" accumST = accumSTFloat #-} {-# RULES "accumST/Double" accumST = accumSTDouble #-} </code></pre> <p>The Generic Unboxed version (which doesn't work) would have a type constraint like the following:</p> <pre><code>accumSTU :: forall a. (IArray UArray a, Num a, forall s. MArray (STUArray s) a (ST s)) => [a] -> Int -> a </code></pre> <p>You could simplify as follows:</p> <pre><code>-- accumST :: forall a. (IArray UArray a, Num a) => [a] -> Int -> a accumST :: forall a. (IArray UArray a, Num a) => [a] -> Int -> a accumST vals = (!) . runSTArray $ do arr <- newArray (0, length vals) 0 :: (Num a) => ST s (STArray s Int a) accumST_inner vals arr accumST_inner vals arr = do forM_ (zip vals [1 .. length vals]) $ \(val, i) -> readArray arr (i - 1) >>= writeArray arr i . (+ val) return arr accumSTFloat vals = (!) . runSTUArray $ do arr <- newArray (0, length vals) 0 :: ST s (STUArray s Int Float) accumST_inner vals arr accumSTDouble vals = (!) . runSTUArray $ do arr <- newArray (0, length vals) 0 :: ST s (STUArray s Int Double) accumST_inner vals arr {-# RULES "accumST/Float" accumST = accumSTFloat #-} {-# RULES "accumST/Double" accumST = accumSTDouble #-} </code></pre>

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