# Ord instance

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Depending on these opinions we come to different conclusions | Depending on these opinions we come to different conclusions | ||

whether there should be <hask>Ord</hask> instances for <hask>Bool</hask> and <hask>Complex</hask> numbers. | whether there should be <hask>Ord</hask> instances for <hask>Bool</hask> and <hask>Complex</hask> numbers. | ||

+ | In most circumstances expressions like <hask>a < b</hask> are certainly a bug, | ||

+ | when <hask>a</hask> and <hask>b</hask> are <hask>Bool</hask> or <hask>Complex</hask> numbers. | ||

+ | Consider someone rewrites an algorithm for real numbers to complex numbers | ||

+ | and he relies on the type system to catch all inconsistencies. | ||

+ | The field operations can remain the same, but <hask>(<)</hask> has to be applied to results of <hask>abs</hask>, | ||

+ | <hask>realPart</hask> or other functions that yield a real. | ||

+ | The truth of <hask>False < True</hask> relies on the encoding of <hask>False</hask> by 0 and <hask>True</hask> by 1. | ||

+ | However there are also programming languages that represent "true" by -1, because this has bit pattern 1....1. | ||

+ | The CPU has an instruction to fill a byte with the content of a flag | ||

+ | and you can use this bit pattern for bitwise AND and OR operations. | ||

+ | This makes that representation very efficient. | ||

+ | In such a language it is <hask>False > True</hask>. | ||

+ | If you use the numeric value of boolean values for arithmetics like in <hask>2 * fromEnum bool - 1</hask> | ||

+ | in order to map <hask>False</hask> to -1 and <hask>True</hask> to 1 without an <hask>if-then-else</hask>, | ||

+ | then porting a program between different representations of boolean values becomes error-prone. | ||

− | + | However you like to work with <hask>Set</hask>s of boolean values and complex numbers, | |

+ | and <hask>Set</hask> requires an <hask>Ord</hask> instance. | ||

− | |||

== See also == | == See also == |

## Revision as of 09:20, 3 September 2010

What is the meaning of theOrd

Ord

However opinions differ whether there shall be more in it:

- An instance may also suggest a notion of magnitudeOrd
- An instance may be free of any other associationOrd

Depending on these opinions we come to different conclusions

whether there should beOrd

Bool

Complex

a < b

a

b

Bool

Complex

Consider someone rewrites an algorithm for real numbers to complex numbers and he relies on the type system to catch all inconsistencies.

The field operations can remain the same, but(<)

abs

realPart

False < True

False

True

However there are also programming languages that represent "true" by -1, because this has bit pattern 1....1. The CPU has an instruction to fill a byte with the content of a flag and you can use this bit pattern for bitwise AND and OR operations. This makes that representation very efficient.

In such a language it isFalse > True

2 * fromEnum bool - 1

False

True

if-then-else

then porting a program between different representations of boolean values becomes error-prone.

However you like to work withSet

Set

Ord

## See also

- Haskell-Cafe on Unnecessarily strict implementations