[Haskell-beginners] Lambda Functions
tom.davie at gmail.com
Thu Feb 26 17:25:42 EST 2009
On 26 Feb 2009, at 23:07, Philip Scott wrote:
> Okay, thanks everyone for the help with the last question; I've got
> another one for you if you are keen. It's short and sweet.
> You can easily define a lambda expression that takes a tuple - e.g.
> in ghci
> > let foo = \(x,y) -> 42
> foo :: (t, t1) -> Integer
> > foo (5,5)
> Yay. Now that isn't a very exciting function. Tt takes two anythings
> and gives you a nice meaningful number. Now let us say for some
> perverse reason I want to make a lambda to test for equality. That
> is, reimplement the functionality of (==) using, well, (==).
> > let foo2 = \(x,y) -> x == y
> foo2 :: ((), ()) -> Bool
> Uh-oh. Now things are getting a little odd. I am not entirely sure
> what that type signature means (I was expecting to see something
> about the types having to be the same and an instance of the Eq
> class but alas..) The function certainly doesn't do what I want it to:
> > foo2 (5,5)
> No instance for (Num ())
> arising from the literal `5' at <interactive>:1:6
> Possible fix: add an instance declaration for (Num ())
> In the expression: 5
> In the first argument of `foo2', namely `(5, 5)'
> In the expression: foo2 (5, 5)
> This is itself a stupid problem, but it is a distilled version of
> some trouble I was having writing a filter that works on a list of
> tuples. Any pointers or slaps in the face for being too stupid
> warmly welcomed. I have already tried my usual trick of smothering
> the thing parentheses but it appeared to be all in vain.
I'm not 100% certain here, so someone may correct me, but I think this
is what's going on:
foo2 has no arguments. Because of this, ghci makes it a CAF. At this
point, the monomorphism restriction kicks in, and the CAF has to be
monomorphic. ghc then chooses a type for it, and defaulting choses
the most simple type it can find – () (this is the type that contains
one value – () ).
If instead, you do let foo3 (x,y) = x == y, you will get the type you
Another way to write it ofc would simply be uncurry (==).
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