| 1 | 11c11,26 |
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| 2 | < -- The 'trace' function. |
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| 3 | --- |
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| 4 | > -- The 'trace' function is intended for printing debug messages. |
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| 5 | > -- Using 'trace', a message is printed as a side effect of evaluating an expression. |
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| 6 | > -- |
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| 7 | > -- Example: |
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| 8 | > -- |
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| 9 | > -- > fib 0 = trace "base 0" 1 |
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| 10 | > -- > fib 1 = trace "base 1" 1 |
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| 11 | > -- > fib n = trace ("fib " ++ show n) (fib (n-1) + fib (n-2)) |
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| 12 | > -- |
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| 13 | > -- Now calculating @fib 3@ will output: |
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| 14 | > -- |
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| 15 | > -- > fib 3 |
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| 16 | > -- > fib 2 |
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| 17 | > -- > base 1 |
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| 18 | > -- > base 0 |
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| 19 | > -- > base 1 |
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| 20 | 16d30 |
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| 21 | < -- * Tracing |
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| 22 | 19c33,36 |
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| 23 | < traceShow |
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| 24 | --- |
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| 25 | > traceShow, |
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| 26 | > withTrace, |
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| 27 | > traceM, |
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| 28 | > traceShowM |
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| 29 | 70a88,107 |
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| 30 | > |
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| 31 | > -- | The expression @withTrace msg x@ prints @msg ++ show x@ before returning @x@. |
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| 32 | > withTrace :: Show a => String -> a -> a |
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| 33 | > withTrace msg x = trace (msg ++ show x) x |
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| 34 | > |
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| 35 | > -- | Output the specified string before returning @()@. This function is intended for tracing in a do-block. |
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| 36 | > -- |
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| 37 | > -- Note: In non-strict monads you need to write |
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| 38 | > -- |
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| 39 | > -- > do () <- traceM msg |
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| 40 | > -- |
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| 41 | > -- to ensure that the trace message is output. |
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| 42 | > traceM :: Monad m => String -> m () |
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| 43 | > traceM msg = trace msg (return ()) |
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| 44 | > |
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| 45 | > -- | Like 'traceM', but uses 'show' on the argument to convert it to a 'String'. |
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| 46 | > -- |
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| 47 | > -- > traceShowM = traceM . show |
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| 48 | > traceShowM :: (Show a, Monad m) => a -> m () |
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| 49 | > traceShowM = traceM . show |
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