Monads/Writer monad: Difference between revisions
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</syntaxhighlight>
=={{header|FreeBASIC}}==
{{trans|Go}}
<syntaxhighlight lang="vbnet">Type mwriter
value As Double
log_ As String
End Type
Function Unit(v As Double, s As String) As mwriter
Dim As mwriter mw
mw.value = v
mw.log_ = " " & s & ": " & Str(v) & Chr(10)
Return mw
End Function
Function Root(mw As mwriter) As mwriter
mw.value = Sqr(mw.value)
mw.log_ = mw.log_ & " Took square Root: " & Str(mw.value) & Chr(10)
Return mw
End Function
Function addOne(mw As mwriter) As mwriter
mw.value = mw.value + 1
mw.log_ = mw.log_ & " Added one : " & Str(mw.value) & Chr(10)
Return mw
End Function
Function Half(mw As mwriter) As mwriter
mw.value = mw.value / 2
mw.log_ = mw.log_ & " Divided by two : " & Str(mw.value) & Chr(10)
Return mw
End Function
Dim As mwriter mw1
mw1 = Unit(5, "Initial value ")
mw1 = Root(mw1)
mw1 = addOne(mw1)
mw1 = Half(mw1)
Print "The Golden Ratio is "; mw1.value
Print !"\nThis was derived as follows:-"
Print mw1.log_
Sleep</syntaxhighlight>
{{out}}
<pre>The Golden Ratio is 1.618033988749895
This was derived as follows:-
Initial value : 5
Took square Root: 2.23606797749979
Added one : 3.23606797749979
Divided by two : 1.618033988749895</pre>
=={{header|F_Sharp|F#}}==
Line 617 ⟶ 668:
added 1 -> 3.23607
divided by 2 -> 1.61803</syntaxhighlight>
=={{header|Java}}==
<syntaxhighlight lang="java">
import java.util.function.Function;
public final class MonadWriter {
public static void main(String[] aArgs) {
Monad<Double> initial = Monad.unit(5.0, "Initial value");
Monad<Double> result = initial.bind(MonadWriter::root).bind(MonadWriter::addOne).bind(MonadWriter::half);
System.out.println("The Golden Ratio is " + result.getValue() + System.lineSeparator());
System.out.println("This was derived as follows:" + System.lineSeparator() + result.getText());
}
private static Monad<Double> root(double aD) {
return Monad.unit(Math.sqrt(aD), "Took square root");
}
private static Monad<Double> addOne(double aD) {
return Monad.unit(aD + 1.0, "Added one");
}
private static Monad<Double> half(double aD) {
return Monad.unit(aD / 2.0, "Divided by two");
}
}
final class Monad<T> {
public static <T> Monad<T> unit(T aValue, String aText) {
return new Monad<T>(aValue, aText);
}
public Monad<T> bind(Function<T, Monad<T>> aFunction) {
Monad<T> monad = aFunction.apply(value);
monad.text = text + monad.text;
return monad;
}
public T getValue() {
return value;
}
public String getText() {
return text;
}
private Monad(T aValue, String aText) {
value = aValue;
text = String.format("%-21s%s%n", " " + aText, ": " + aValue);
}
private T value;
private String text;
}
</syntaxhighlight>
{{ out }}
<pre>
The Golden Ratio is 1.618033988749895
This was derived as follows:
Initial value : 5.0
Took square root : 2.23606797749979
Added one : 3.23606797749979
Divided by two : 1.618033988749895
</pre>
=={{header|JavaScript}}==
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=={{header|Python}}==
<syntaxhighlight lang="python">
"""A Writer Monad. Requires Python >= 3.7 for type hints."""
from __future__ import annotations
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import os
from typing import Callable
from typing import Generic
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T = TypeVar("T")
U = TypeVar("U")
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self.msgs = list(f"{msg}: {self.value}" for msg in msgs)
def bind(self, func: Callable[[T], Writer[
writer = func(self.value)
return Writer(writer, *self.msgs)
def __rshift__(self, func: Callable[[T], Writer[
return self.bind(func)
Line 1,101 ⟶ 1,221:
def lift(func: Callable[[T], U], msg: str) -> Callable[[
"""Return a writer monad version of the simple function `func`."""
@functools.wraps(func)
def wrapped(value: T) -> Writer[U]:
return Writer(func(value), msg)
Line 1,113 ⟶ 1,233:
if __name__ == "__main__":
square_root = lift(math.sqrt, "square root")
add_one: Callable[[Union[int, float]], Writer[Union[int, float]]] = lift(
lambda x: x + 1, "add one"
)
half: Callable[[Union[int, float]], Writer[float]] = lift(
lambda x: x / 2, "div two"
)
print(Writer(5, "initial") >> square_root >> add_one >> half)
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{{trans|Go}}
{{libheader|Wren-fmt}}
<syntaxhighlight lang="
class Mwriter {
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