[Haskell-cafe] pattern matches are overlapped

[bri...@aracnet.com]

This works fine

keyboardAct a d p (SpecialKey KeyLeft) Down Modifiers {shift=Down, ctrl=Up, alt=Up} = do
(x,y) <- get p
p $= (x-0.1, y)

This

keyboardAct a d p (SpecialKey KeyLeft) Down shiftDown = do
(x,y) <- get p
p $= (x-0.1, y)

where,

shiftDown = Modifiers {shift=Down, ctrl=Up, alt=Up}

gives me :

KeyBindings.hs:32:1: Warning:
Pattern match(es) are overlapped
In an equation for `keyboardAct':
keyboardAct angle
delta
p
(SpecialKey KeyLeft)
Down
Modifiers {shift = Up, ctrl = Up, alt = Up}
=
...

keyboardAct is used several times via pattern matching, e.g.

keyboardAct angle delta p (SpecialKey KeyLeft) Down Modifiers {shift=Up, ctrl=Up, alt=Up} = do
(x,y) <- get p
a <- get angle
d <- get delta
angle $= a+d

and there are several more.

So clearly, the pattern match is overlapped, just like the warning says.

The question is why I don't get the same warning before I make use of shiftDown.

Thanks
Brian
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[Brandon Allbery]

On Thu, Oct 23, 2014 at 2:23 PM, <bri...@aracnet.com> wrote:

keyboardAct a d p (SpecialKey KeyLeft) Down shiftDown = do
  (x,y) <- get p
  p $= (x-0.1, y)

where,

   shiftDown = Modifiers {shift=Down, ctrl=Up, alt=Up}

You can't use a definition like that directly in a pattern match; a name starting with lowercase is a new binding, not the value of some existing binding. You can instead use a guard or, sometimes, a pattern guard.

That is,

    keyboardAct a d p (SpecialKey KeyLeft) Down shiftDown =

is identical to

    keyboardAct a d p (SpecialKey KeyLeft) Down x =

aside from the name of the new local binding it introduces. It does *not* use an existing binding for shiftDown or x, it always creates a new one.

--

brandon s allbery kf8nh                               sine nomine associates

allb...@gmail.com                                  ball...@sinenomine.net

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