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Today's Topics:
1. Re: pattern matching on function names or algebraic type
data constructors (Michael Peternell)
2. Re: pattern matching on function names or algebraic type
data constructors (Daniel Trstenjak)
----------------------------------------------------------------------
Message: 1
Date: Sun, 23 Jun 2013 13:08:48 +0200
From: Michael Peternell <[email protected]>
Subject: Re: [Haskell-beginners] pattern matching on function names or
algebraic type data constructors
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
I think the deeper meaning of the error message is that functions are not part
of the Eq type class. So you cannot really compare functions. Think about it: 2
functions are the same (mathematically) if they have the same signature and if
they return the same value for each legal input. This is computationally not
decidable. We could compare function identity, but that would be in contrast to
the "referential transparency principle" of Haskell. I'm wondering why the code
even compiles though.
Code example:
Prelude> (const 4) == (const 5)
<interactive>:5:11:
No instance for (Eq (b0 -> a0))
arising from a use of `=='
Possible fix: add an instance declaration for (Eq (b0 -> a0))
In the expression: (const 4) == (const 5)
In an equation for `it': it = (const 4) == (const 5)
-Michael
Am 23.06.2013 um 11:34 schrieb TP <[email protected]>:
> Hi,
>
> I ask myself if there is a way to make the following code work.
> I would like to pattern match on function names:
>
> -----------
> f :: Float -> Int
> f x = 1
>
> g :: Float -> Int
> g x = 2
>
> pat :: (Float -> Int) -> Bool
> pat t = case t of
> f -> True
> g -> False
>
> main = do
>
> print $ pat f
> print $ pat g
> -----------
>
> I don't understand the output:
>
> -----------
> $ runghc test_pattern_match_on_function_2.hs
>
> test_pattern_match_on_function_2.hs:8:9: Warning:
> Pattern match(es) are overlapped
> In a case alternative: g -> ...
> True
> True
> -----------
>
> What is the reason for this message?
>
> Now, I want to do it on data constructors in a GADT (which are functions if
> I am right):
> -----------
> {-# LANGUAGE GADTs #-}
>
> data Foobar where
> Mult :: Float -> Foobar
> Plus :: Float -> Foobar
>
> pat :: (Float -> Foobar) -> Bool
> pat t = case t of
> Mult -> True
> Plus -> False
>
> main = do
>
> print $ pat Mult
> print $ pat Plus
> -----------
>
> I obtain:
>
> -----------
> $ runghc test_pattern_match_on_function.hs
>
> test_pattern_match_on_function.hs:9:9:
> Constructor `Mult' should have 1 argument, but has been given none
> In the pattern: Mult
> In a case alternative: Mult -> True
> In the expression:
> case t of {
> Mult -> True
> Plus -> False }
>
> test_pattern_match_on_function.hs:9:9:
> Couldn't match expected type `Float -> Foobar'
> with actual type `Foobar'
> In the pattern: Mult
> In a case alternative: Mult -> True
> In the expression:
> case t of {
> Mult -> True
> Plus -> False }
> -----------
>
> Why does it not work? If Mult and Plus are functions, then I should not need
> to put their arguments in pattern matching.
>
> Thanks in advance,
>
> TP
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
------------------------------
Message: 2
Date: Sun, 23 Jun 2013 14:43:51 +0200
From: Daniel Trstenjak <[email protected]>
Subject: Re: [Haskell-beginners] pattern matching on function names or
algebraic type data constructors
To: [email protected]
Message-ID: <20130623124351.GA6412@machine>
Content-Type: text/plain; charset=us-ascii
Hi TP,
> What is the reason for this message?
'f' and 'g' in the 'case' expression are local names/bindings
and have nothing in common with the 'f' and 'g' functions.
It's the same like introducing names/bindings with the 'let' expression:
let f = ...
g = ...
in ...
The bindings are overlapping, because both match all the time and
GHC will always use the first one.
> Why does it not work? If Mult and Plus are functions, then I should not need
> to put their arguments in pattern matching.
'Mult' and 'Plus' are data constructors which both expect one argument: a
'Float'.
You also can't match the data constructors against a function, but only
against a value of its type.
pat :: Foobar -> Bool
pat t = case t of
Mult f -> True
Plus f -> False
Greetings,
Daniel
------------------------------
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