This is a project in which I implemented a calculator in Haskell.
It uses parse combinators to parse the users input and then evaluates the expression and prints the result.
Ensure that you have Stack installed.
Then, to build the project run
stack build
In the top level directory of this repo.
Then to run it (at present I do not have a resiliant input method yet) run
stack run "2 + 3"
Replace 2 + 3 with whatever mathematical expression you like that is built only of +, -, *, /, and constants.
This will parse your inputs using the standard order of operations, that is 2 + 3 * 4 means the same thing as
2 + (3 * 4) and is not read left-to-right as (2 + 3) * 4.
Note that Haskell by default uses % to mean 2 % 3 is the Ratio 2/3. It will bind tighter than any other option.
Expr is (2 % 1 + 3 % 1)
result when computed as a Double is 5.0
result computed as a Rational is = 5 % 1
result computed as a Rational and casted to a Double is = 5.0
Expr is (2 % 1 + 3 % 1)
result when computed as a Double is 5.0
result computed as a Rational is = 5 % 1
result computed as a Rational and casted to a Double is = 5.0
Expr is ((1 % 1000000000000000000000 + 1000000000 % 1) - 1000000000 % 1)
result when computed as a Double is 0.0
result computed as a Rational is = 1 % 1000000000000000000000
result computed as a Rational and casted to a Double is = 1.0e-21
Note that this example suffers if done as a double (as seen in GHCi)
Prelude> 0.000000000000000000001 + 1000000000 - 1000000000 :: Double
0.0
Expr is (2 % 1 + (3 % 1 * ((2 % 1 / 3 % 1) - (4 % 1 / 5 % 1))))
result when computed as a Double is 1.5999999999999996
result computed as a Rational is = 8 % 5
result computed as a Rational and casted to a Double is = 1.6
Expr is (3 % 10 - 1 % 5)
result when computed as a Double is 9.999999999999998e-2
result computed as a Rational is = 1 % 10
result computed as a Rational and casted to a Double is = 0.1
In GHCi to show what would normally happen with this computation.
Prelude> 0.3 - 0.2 :: Double
9.999999999999998e-2
Error when parsing: (interactive):1:7: error: unexpected
EOF, expected: ")", "*", "+",
"-", ".", "/", digit, exponent
(2 + 3<EOF>
^
Here we can see how the parser can explain what went wrong with the parse. Not just that the input was not understandable.
Error when parsing: (interactive):1:5: error: expected: Factor
2 + a + 3<EOF>
^
And this even points at errors in the middle of a parse.