Pattern match is redundant

Question

Haskell, Stack build tool.

I have code:

quote :: Char
quote = '\''

doubleQuote :: Char
doubleQuote = '\"'

isBorder :: Char -> Bool
isBorder quote = True
isBorder doubleQuote = True
isBorder _ = False

It will be compiled without erors, but I see the messages during the compilation:

D:\haskell\real\app\Main.hs:34:1: warning: [-Woverlapping-patterns]
Pattern match is redundant
In an equation for `isBorder': isBorder doubleQuote = ...

D:\haskell\real\app\Main.hs:35:1: warning: [-Woverlapping-patterns]
Pattern match is redundant
In an equation for `isBorder': isBorder _ = ...

What does it mean? I don't see redundance...

Solution 1

When declaring functions, the argument variables are new names. Your quote and doubleQuote are shadowing the functions rather than invoking the functions for pattern matching. In this way the language of pattern matching subtly deviates from the language of right-hand-sided expressions. To achieve what you want, either do

isBorder :: Char -> Bool
isBorder '\'' = True
isBorder '\"' = True
isBorder _ = False

or

isBorder :: Char -> Bool
isBorder c | quote == c = True
isBorder c | doubleQuote == c = True
isBorder _ = False

The philosophy is to first bind the argument to name and then to use a guard to invoke an expression that evaluates to a boolean.

Solution 2

The pattern can only be matched against concrete values, not against identifiers.

So the compiler essentially sees:

isBorder x = True
isBorder x = True

since quote and doublequote aren't the defined identifers but the names of the parameters.

Comments

[ThreeFx]

You can pack all the calls in one top-level function.

[Andrey Bushman]

> since quote and doublequote aren't the defined identifers but the names of the parameters. Hm... But I see that my code works right... So Haskell understands what I meaned in my definitions.

[Andrey Bushman]

Yes, I understand you and I think you are right. Hm... But I see that my code works right too... So Haskell understands what I meaned in my definitions. Look my full code example, please here: yadi.sk/d/MfyG7OmHwGmsV

[ThreeFx]

Really? Is isBorder '5' false?