Group items tagged function - Haskell

shared by Javier Neira on 24 Jan 10 - Cached

The most interesting are probably the static checks that can be done with Type arithmetic, that enforce the number of bands at compile time, rather than runtime, by lifting the band count into the type level.
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typecheck perform the check statically, using phantom types and Peano numbers.
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So encode a type-level version of the bounds check. Only resistors with bands >= 4 and <= 8 are valid:
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...1 more annotation...
Further checks can be obtained by separating the metal and ceramic values on the type level, so no function that takes a metal resistor can be accidentally passed a ceramic one. A newtype is useful for this:
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shared by J.A. Alonso on 24 Jan 10 - Cached

ap module: Control.Monad type: ap :: (Monad m) => m (a -> b) -> m a -> m b
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shared by J.A. Alonso on 14 Sep 11 - No Cached

shared by Javier Neira on 09 Nov 09 - Cached

type Choice a = [a] choose :: [a] -> Choice a choose xs = xs
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Because Haskell doesn’t compute answers until we ask for them, we get the actual backtracking for free!
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The missing function is almost too trivial to mention: Given a single value of type a, we need a convenient way to construct a value of type Choice a:
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More math trivia: return is also known as unit and η. That’s a lot of names for a very simple idea.)
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makePairs = choose [1,2,3] >>= (\x -> choose [4,5,6] >>= (\y -> return (x,y)))
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makePairs' = do x <- choose [1,2,3] y <- choose [4,5,6] return (x,y)
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Every monad has three pieces: return, map and join.
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Backtracking: The lazy way to code
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shared by Javier Neira on 10 Nov 09 - Cached

16.2  Incremental Array Updates The operator (//) takes an array and a list of pairs and returns an array identical to the left argument except that it has been updated by the associations in the right argument. (As with the array function, the indices in the association list must be unique for the updated elements to be defined.) For example, if m is a 1-origin, n by n matrix, then m//[((i,i), 0) | i <- [1..n]] is the same matrix, except with the diagonal zeroed.
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-- A rectangular subarray subArray :: (Ix a) => (a,a) -> Array a b -> Array a b subArray bnds = ixmap bnds (\i->i) -- A row of a matrix row :: (Ix a, Ix b) => a -> Array (a,b) c -> Array b c row i x = ixmap (l',u') (\j->(i,j)) x where ((_,l'),(_,u')) = bounds x -- Diagonal of a matrix (assumed to be square) diag :: (Ix a) => Array (a,a) b -> Array a b diag x = ixmap (l,u) (\i->(i,i)) x        where           ((l,_),(u,_)) = bounds x -- Projection of first components of an array of pairs firstArray :: (Ix a) => Array a (b,c) -> Array a b firstArray = fmap (\(x,y)->x)
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