Haskell: GHCi: Safely ignore warnings while running

Consider following program, lengthOfList function takes list of any type and return number of elements in the list. Don’t worry much about the program, I will explain about this in my later posts.

ListUtil.hs

lengthOfList :: [a] -> Integer
lengthOfList [] = 0
lengthOfList (x:xs) = 1 + lengthOfList xs

Prelude> :load ListUtil.hs
[1 of 1] Compiling Main             ( ListUtil.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> lengthOfList "abcd"
4

Try to run lengthOfList function on list of numbers, If warnings are enabled on your machine, you will get following kind of output.

*Main> lengthOfList [1, 2, 3]

<interactive>:9:15: Warning:
    Defaulting the following constraint(s) to type ‘Integer’
      (Num a0) arising from the literal ‘1’
    In the expression: 1
    In the first argument of ‘lengthOfList’, namely ‘[1, 2, 3]’
    In the expression: lengthOfList [1, 2, 3]

<interactive>:9:15: Warning:
    Defaulting the following constraint(s) to type ‘Integer’
      (Num a0) arising from the literal ‘1’
    In the expression: 1
    In the first argument of ‘lengthOfList’, namely ‘[1, 2, 3]’
    In the expression: lengthOfList [1, 2, 3]
3

Warning is telling you that Integer is the default choice, Same will happen to real numbers also.

*Main> lengthOfList [1.1, 2.2, 3.3]

<interactive>:11:15: Warning:
    Defaulting the following constraint(s) to type ‘Double’
      (Fractional a0) arising from the literal ‘1.1’
    In the expression: 1.1
    In the first argument of ‘lengthOfList’, namely ‘[1.1, 2.2, 3.3]’
    In the expression: lengthOfList [1.1, 2.2, 3.3]

<interactive>:11:15: Warning:
    Defaulting the following constraint(s) to type ‘Double’
      (Fractional a0) arising from the literal ‘1.1’
    In the expression: 1.1
    In the first argument of ‘lengthOfList’, namely ‘[1.1, 2.2, 3.3]’
    In the expression: lengthOfList [1.1, 2.2, 3.3]
3

Warning is telling you, Double is the default choice.

How to disable above warnings ?

There are two ways.

a. Use the command ‘:set –w’

*Main> :set -w
*Main> lengthOfList [1.1, 2.2, 3.3]
3
*Main> lengthOfList [1, 2, 3]
3

b. Open GHCi prompt using ‘ghci -fno-warn-type-defaults’

$ ghci -fno-warn-type-defaults
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
Prelude> 
Prelude> :load ListUtil.hs
[1 of 1] Compiling Main             ( ListUtil.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> lengthOfList [1, 2, 3]
3
*Main> lengthOfList [1.1, 2.2, 3.3]
3

If you want to see the warnings use ‘:set –Wall’.

*Main> :set -Wall
*Main> 
*Main> lengthOfList [1.1, 2.2, 3.3]

<interactive>:10:15: Warning:
    Defaulting the following constraint(s) to type ‘Double’
      (Fractional a0) arising from the literal ‘1.1’
    In the expression: 1.1
    In the first argument of ‘lengthOfList’, namely ‘[1.1, 2.2, 3.3]’
    In the expression: lengthOfList [1.1, 2.2, 3.3]

<interactive>:10:15: Warning:
    Defaulting the following constraint(s) to type ‘Double’
      (Fractional a0) arising from the literal ‘1.1’
    In the expression: 1.1
    In the first argument of ‘lengthOfList’, namely ‘[1.1, 2.2, 3.3]’
    In the expression: lengthOfList [1.1, 2.2, 3.3]
3

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