# 99 questions/Solutions/46

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− | -- | + | -- NOT negates a single Boolean argument |

not' :: Bool -> Bool | not' :: Bool -> Bool | ||

not' True = False | not' True = False | ||

not' False = True | not' False = True | ||

− | -- | + | -- Type signature for remaining logic functions |

and',or',nor',nand',xor',impl',equ' :: Bool -> Bool -> Bool | and',or',nor',nand',xor',impl',equ' :: Bool -> Bool -> Bool | ||

+ | |||

+ | -- AND is True if both a and b are True | ||

and' True True = True | and' True True = True | ||

and' _ _ = False | and' _ _ = False | ||

− | -- True if a or b or both are True | + | -- OR is True if a or b or both are True |

or' False False = False | or' False False = False | ||

or' _ _ = True | or' _ _ = True | ||

− | -- | + | -- NOR is the negation of 'or' |

nor' a b = not' $ or' a b | nor' a b = not' $ or' a b | ||

− | -- | + | -- NAND is the negation of 'and' |

nand' a b = not' $ and' a b | nand' a b = not' $ and' a b | ||

− | -- True if either a or b is | + | -- XOR is True if either a or b is True, but not if both are True |

xor' True False = True | xor' True False = True | ||

xor' False True = True | xor' False True = True | ||

xor' _ _ = False | xor' _ _ = False | ||

− | -- True if a implies b, equivalent to (not a) or (b) | + | -- IMPL is True if a implies b, equivalent to (not a) or (b) |

impl' a b = (not' a) `or'` b | impl' a b = (not' a) `or'` b | ||

− | -- True if a and b are equal | + | -- EQU is True if a and b are equal |

equ' True True = True | equ' True True = True | ||

equ' False False = True | equ' False False = True | ||

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− | The | + | The above implementations build each logic function from scratch; they could be shortened using Haskell's builtin equivalents: |

<haskell> | <haskell> | ||

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</haskell> | </haskell> | ||

− | Some could be reduced even further | + | Some could be reduced even further using [[Pointfree]] style: |

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− | + | The only remaining task is to generate the truth table; most of the complexity here comes from the string conversion and IO. The approach used here accepts a Boolean function <tt>(Bool -> Bool -> Bool)</tt>, then calls that function with all four combinations of two Boolean values, and converts the resulting values into a list of space-separated strings. Finally, the strings are printed out by mapping <hask>putStrLn</hask> across the list of strings: | |

<haskell> | <haskell> |

## Revision as of 04:07, 18 July 2010

(**) Define predicates and/2, or/2, nand/2, nor/2, xor/2, impl/2 and equ/2 (for logical equivalence) which succeed or fail according to the result of their respective operations; e.g. and(A,B) will succeed, if and only if both A and B succeed.

A logical expression in two variables can then be written as in the following example: and(or(A,B),nand(A,B)).

Now, write a predicate table/3 which prints the truth table of a given logical expression in two variables.

The first step in this problem is to define the Boolean predicates:

-- NOT negates a single Boolean argument not' :: Bool -> Bool not' True = False not' False = True -- Type signature for remaining logic functions and',or',nor',nand',xor',impl',equ' :: Bool -> Bool -> Bool -- AND is True if both a and b are True and' True True = True and' _ _ = False -- OR is True if a or b or both are True or' False False = False or' _ _ = True -- NOR is the negation of 'or' nor' a b = not' $ or' a b -- NAND is the negation of 'and' nand' a b = not' $ and' a b -- XOR is True if either a or b is True, but not if both are True xor' True False = True xor' False True = True xor' _ _ = False -- IMPL is True if a implies b, equivalent to (not a) or (b) impl' a b = (not' a) `or'` b -- EQU is True if a and b are equal equ' True True = True equ' False False = True equ' _ _ = False

The above implementations build each logic function from scratch; they could be shortened using Haskell's builtin equivalents:

and' a b = a && b or' a b = a || b nand' a b = not (and' a b) nor' a b = not (or' a b) xor' a b = and' (or' a b) (nand' a b) impl' a b = or' (not a) b equ' a b = a == b

Some could be reduced even further using Pointfree style:

and' = (&&) or' = (||) equ' = (==)

`(Bool -> Bool -> Bool)`, then calls that function with all four combinations of two Boolean values, and converts the resulting values into a list of space-separated strings. Finally, the strings are printed out by mapping

table :: (Bool -> Bool -> Bool) -> IO () table f = mapM_ putStrLn [show a ++ " " ++ show b ++ " " ++ show (f a b) | a <- [True, False], b <- [True, False]]

The table function in Lisp supposedly uses Lisp's symbol handling to substitute variables on the fly in the expression. I chose passing a binary function instead because parsing an expression would be more verbose in haskell than it is in Lisp. Template Haskell could also be used :)