On 07.01.2010, at 02:23, Steven E. Harris wrote:
That means that a monadic function has a signature like
a -> m b
RIght.
Say that we're looking to use some "normal" functions with this
monad. Those functions may have signatures like
a -> b
They clearly don't return the right kind of value. There must be some
way to integrate such functions without writing new wrappers around
them
by hand. Is this a job for `m-fmap'?
There are various ways to construct monadic values. m-fmap is one of
them, but it already requires a monadic value as input. The simplest
way to construct a monadic value is m-result.
Reading the implementation, it
looks like it would take a "normal" function and allow it to call on
the
basic value extracted from a monadic value.
Indeed. One way to define m-fmap is
(defn m-fmap [f mv]
(domonad
[x mv]
(f x)))
But note that the second value is already a monadic value.
The lift operator also sounded relevant, but, if I understand it
correctly, it converts a "normal" function to one that accepts a
monadic
value and returns a monadic value.
Right. For a function of a single argument, m-lift and m-fmap are
equivalent.
That's the wrong argument type to be used with bind -- which wants
to call on a monadic function with a basic value -- so I don't
understand when one would want to use lift. When
would one be able to call on a "lifted" function? A simple example
would
help.
Here is an example taken from clojure.contrib.monads.examples. The
function "pairs" take a sequence of values and produces a sequences of
all pairs of items in the input sequence:
(with-monad sequence-m
(defn pairs [xs]
((m-lift 2 #(list %1 %2)) xs xs)))
Konrad.
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