Haskell: Enable warnings at GHCi prompt

By calling ‘:set –Wall’ at ghci prompt, you can enable warnings at GHCi prompt.

Fact.hs

factorial :: Integer -> Integer
factorial n
    | n == 0 = 1
    | n /= 0 = n * factorial (n-1)

Load above program into GHCi prompt.

Prelude> :load Fact.hs 
[1 of 1] Compiling Main             ( Fact.hs, interpreted )
Ok, modules loaded: Main.
*Main> 

Now enable warnings using ‘:set –Wall’ command and reload Fact.hs.

*Main> :set -Wall
*Main> 
*Main> :load Fact.hs 
[1 of 1] Compiling Main             ( Fact.hs, interpreted )

Fact.hs:2:1: Warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘factorial’: Patterns not matched: _
Ok, modules loaded: Main.

How to disable warnings?

Use ‘:set –w’ to disable warnings.

*Main> :set -w
*Main> 
*Main> :load Fact.hs 
[1 of 1] Compiling Main             ( Fact.hs, interpreted )
Ok, modules loaded: Main.

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