ML - datatype and functions
We have the next dayatype:
datatype complex = Rec of real * real | Polar of real * real;
and two functions:
- val real =
fn (Rec(x,y) ) => x
| (Polar(r,a)) => r * Math.cos(a);
val real = fn : complex -> real
- val imaginary =
fn (Rec(x,y) ) => y
| (Polar(r,a)) => r * Math.sin(a);
val imaginary = fn : complex -> real
Now, the book defined another function:
- val add_complex =
fn (Rec(x, y), Rec(x', y')) => ( Rec( x + x', y + y') )
| (Rec(x,y), z) => ( Rec( x + real(z), y + imaginary(z) ) )
| (z, Rec(x, y)) => ( 开发者_运维技巧Rec( real(z) + x, imaginary(z) + y) )
| (z,z') => (Rec( real(z) + real(z'), imaginary(z) + imaginary(z') ) );
val add_complex = fn : complex * complex -> complex
I didn't understand what is the z
in the function add_complex
.
Is it the Polar (meaning, I can write Z=polar(a,b)? If it is, so how the complier know it? meaning - Is it get a z, and parse it to polar variable?
If it is not polar, So what it can be?
In your code, both z
and z'
are Polar
because the first case covers all of the possibilities in which both are Rec
, so in the second case z
is not Rec
, or it would have used the first case. Similarly in the other cases, each z
and z'
must be Polar
because otherwise it would have been caught by a previous case instead. Thus you can safely write z=Polar(a,b)
, or more accurately z=Polar(r, a)
for radius and angle.
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