Why doesn't
1.0 = 2.0
work? Isn't real an equality type?It gives the error:
Error: operator and operand don't agree [equality type required] operator domain: ''Z * ''Z operand: real * real in expression: 1.0 = 2.0
Why won't reals in patterns work like so?
fun fact 0.0 = 1.0 | fact x = x * fact (x - 1.0)
It gives the error:
Error: syntax error: inserting EQUALOP
Why can't I compare reals in Standard ML?
3.5k views Asked by sshine At
1
There are 1 answers
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No. The type variable
''Z
indicates that the operands of=
must have equality types.Pattern matching relies implicitly on testing for equality. The cryptic error message
syntax error: inserting EQUALOP
indicates that the SML/NJ parser does not allow for floating-point literals where a pattern is expected, and so the programmer is prevented from receiving a more meaningful type error.To elaborate,
From http://www.smlnj.org/doc/FAQ/faq.txt:
From http://mlton.org/PolymorphicEquality:
For example,
Real.== (0.1 + 0.2, 0.3)
isfalse
.From http://sml-family.org/Basis/real.html:
The short version: Don't compare reals using equality. Perform an epsilon test. I would recommend reading the article on http://floating-point-gui.de/errors/comparison. In summary:
Don't check if reals are the same, but if the difference is very small.
The error margin that the difference (delta) is compared to is often called epsilon.
Don't compare the difference against a fixed epsilon:
Don't just compare the relative difference against epsilon:
Look out for edge cases:
When
b = 0.0
it raisesDiv
. (Switchinga
andb
provides a symmetric edge case.)When
a
andb
are on opposite sides of zero it returnsfalse
even when they’re the smallest possible non-zero numbers.The result is not commutative. There are cases where
nearlyEqual (a, b, eps)
does not give the same result asnearlyEqual (b, a, eps)
.The guide provides a generic solution; translated to Standard ML this looks like:
And it continues to warn of some edge cases: