Implement, and write tests for, subtraction.
catseye
13 years ago
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> macroP = predP isMacro
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> numberP = predP isNumber
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> robinSubtract env ienv (Pair xexpr (Pair yexpr Null)) cc = do
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> eval env ienv xexpr (\x ->
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> case x of
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> Number xv -> eval env ienv yexpr (\y ->
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> case y of
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> Number yv -> cc (Number (xv - yv))
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> other -> raise ienv (Pair (Symbol "expected-number") other))
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> other -> raise ienv (Pair (Symbol "expected-number") other))
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> robinSubtract env ienv other cc = raise ienv (Pair (Symbol "illegal-arguments") other)
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> robinIf env ienv (Pair test (Pair texpr (Pair fexpr Null))) cc = do
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> eval env ienv test (\x ->
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> case x of
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> ("macro?", macroP),
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> ("number?", numberP),
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> ("equal?", equalP),
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> -- ("subtract", subtract),
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> ("subtract", robinSubtract),
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> -- ("divide", divide),
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> -- ("sign", sign),
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> ("macro", macro),
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| 0 | 0 |
#!/bin/sh
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ghc Main.lhs -o bin/robin
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ghc --make Main.lhs -o bin/robin
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rm -f *.o *.hi Robin/*.o Robin/*.hi
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| (robin (0 . 1) (core (0 . 1))
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| (number? 6 4))
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? robin: uncaught exception: (illegal-arguments 6 4)
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### `subtract` ###
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`subtract` evaluates its first argument to a rational number, then
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evaluates its second argument to a rational number, then evaluates
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to the difference between the first and second numbers.
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| (robin (0 . 1) (core (0 . 1))
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| (subtract 6 4))
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= 2
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| (robin (0 . 1) (core (0 . 1))
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| (subtract 16/15 1/5))
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= 13/15
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| (robin (0 . 1) (core (0 . 1))
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| (subtract 1000 8000))
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= -7000
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Addition may be accomplished by negating the second argument.
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| (robin (0 . 1) (core (0 . 1))
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| (subtract 999 (subtract 0 999)))
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= 1998
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`subtract` expects both of its arguments to be numbers.
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| (robin (0 . 1) (core (0 . 1))
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| (subtract #f 100))
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? robin: uncaught exception: (expected-number . #f)
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| (robin (0 . 1) (core (0 . 1))
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| (subtract 100 ()))
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? robin: uncaught exception: (expected-number)
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`subtract` expects exactly two arguments.
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| (robin (0 . 1) (core (0 . 1))
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| (subtract 100 200 300))
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? robin: uncaught exception: (illegal-arguments 100 200 300)
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| (robin (0 . 1) (core (0 . 1))
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| (subtract))
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? robin: uncaught exception: (illegal-arguments)
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### `eval` ###
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