#Use zip

Often you need to map over a list and apply some function which depends on the index of the argument in the list. while a lot of impure languages who have some sort of map builtin have the index be an optional argument, this is impossible in Haskell. instead, use:

mapWithIndex f xs === f<$>zip[0..]xs
                  === [f i x|(i,x)<-zip[0..]xs] {- inlinable version -}
                  === zipWith f[0..]xs
mapWithIndex f    === (f<$>).zip[0..]           {- points free version -}

(This also gives us 1-based indexing for free!)

Often this combines well within list comprehensions, where even a builtin mapWithIndex won't help:

    [ ... | ..., (i,x)<-zip[0..]xs, ...]

Other times, you really want to use the nonexistant equivalent maximumOn of sortOn, but the import is too many bytes, or using maximumBy is too many bytes too. instead, use*:

sortOn f xs === snd$sort$(f>>=(,))<$>xs
            === snd$sort[(f x,x)|x<-xs]   {- inlinable version -}
sortOn f    === snd.sort.(f>>=(,)<$>)     {- points free version -}

Note that sometimes you will need both the best x and its f x, in which case you can get rid of three bytes and have it computed for you for free!

many other uses for this combination are possible too.

Use zip

Often you need to map over a list and apply some function which depends on the index of the argument in the list. while a lot of impure languages who have some sort of map builtin have the index be an optional argument, this is impossible in Haskell. instead, use:

mapWithIndex f xs === f<$>zip[0..]xs
                  === [f i x|(i,x)<-zip[0..]xs] {- inlinable version -}
                  === zipWith f[0..]xs
mapWithIndex f    === (f<$>).zip[0..]           {- points free version -}

(This also gives us 1-based indexing for free!)

Often this combines well within list comprehensions, where even a builtin mapWithIndex won't help:

    [ ... | ..., (i,x)<-zip[0..]xs, ...]

Other times, you really want to use the nonexistant equivalent maximumOn of sortOn, but the import is too many bytes, or using maximumBy is too many bytes too. instead, use*:

sortOn f xs === snd$sort$(f>>=(,))<$>xs
            === snd$sort[(f x,x)|x<-xs]   {- inlinable version -}
sortOn f    === snd.sort.(f>>=(,)<$>)     {- points free version -}

Note that sometimes you will need both the best x and its f x, in which case you can get rid of three bytes and have it computed for you for free!

many other uses for this combination are possible too.

#Use zip

Often you need to map over a list and apply some function which depends on the index of the argument in the list. while a lot of impure languages who have some sort of map builtin have the index be an optional argument, this is impossible in Haskell. instead, use:

mapWithIndex f xs === f<$>zip[0..]xs
                  === [f i x|(i,x)<-zip[0..]xs] {- inlinable version -}
                  === zipWith f[0..]xs
mapWithIndex f    === (f<$>).zip[0..]           {- points free version -}

(This also gives us 1-based indexing for free!)

Often this combines well within list comprehentions, where even a builtin mapWithIndex won't help:

    [ ... | ..., (i,x)<-zip[0..]xs, ...]

Other times, you really want to use the nonexistant equivalent maximumOn of sortOn, but the import is too many bytes, or using maximumBy is too many bytes too. instead, use*:

sortOn f xs === snd$sort$(f>>=(,))<$>xs
            === snd$sort[(f x,x)|x<-xs]   {- inlinable version -}
sortOn f    === snd.sort.(f>>=(,)<$>)     {- points free version -}

Note that sometimes you will need both the best x and its f x, in which case you can get rid of three bytes and have it computed for you for free!

many other uses for this combination are possible too.

#Use zip

Often you need to map over a list and apply some function which depends on the index of the argument in the list. while a lot of impure languages who have some sort of map builtin have the index be an optional argument, this is impossible in Haskell. instead, use:

mapWithIndex f xs === f<$>zip[0..]xs
                  === [f i x|(i,x)<-zip[0..]xs] {- inlinable version -}
                  === zipWith f[0..]xs
mapWithIndex f    === (f<$>).zip[0..]           {- points free version -}

(This also gives us 1-based indexing for free!)

Often this combines well within list comprehensions, where even a builtin mapWithIndex won't help:

    [ ... | ..., (i,x)<-zip[0..]xs, ...]

Other times, you really want to use the nonexistant equivalent maximumOn of sortOn, but the import is too many bytes, or using maximumBy is too many bytes too. instead, use*:

sortOn f xs === snd$sort$(f>>=(,))<$>xs
            === snd$sort[(f x,x)|x<-xs]   {- inlinable version -}
sortOn f    === snd.sort.(f>>=(,)<$>)     {- points free version -}

Note that sometimes you will need both the best x and its f x, in which case you can get rid of three bytes and have it computed for you for free!

many other uses for this combination are possible too.

#Use zip

Often you need to map over a list and apply some function which depends on the index of the argument in the list. while a lot of impure languages who have some sort of map builtin have the index be an optional argument, this is impossible in Haskell. instead, use:

mapWithIndex f xs === f<$>zip[0..]xs
                  === [f i x|(i,x)<-zip[0..]xs] {- inlinable version -}
                  === zipWith f[0..]xs
mapWithIndex f    === (f<$>).zip[0..]           {- points free version -}

(This also gives us 1-based indexing for free!)

Often this combines well within list comprehentions, where even a builtin mapWithIndex won't help:

    [ ... | ..., (i,x)<-zip[0..]xs, ...]

Other times, you really want to use or the nonexistant equivalent maximumOn of sortOn, but the import is too many bytes, or using maximumBy is too many bytes too. instead, use:

sortOn f xs === snd$sort$(f>>=(,))<$>xs
            === snd$sort[(f x,x)|x<-xs]   {- inlinable version -}
sortOn f    === snd.sort.(f>>=(,)<$>)     {- points free version -}

Note that sometimes you will need both the best x and its f x, in which case you can get rid of three bytes and have it computed for you for free!

many other uses for this combination are possible too.

#Use zip

Often you need to map over a list and apply some function which depends on the index of the argument in the list. while a lot of impure languages who have some sort of map builtin have the index be an optional argument, this is impossible in Haskell. instead, use:

mapWithIndex f xs === f<$>zip[0..]xs
                  === [f i x|(i,x)<-zip[0..]xs] {- inlinable version -}
                  === zipWith f[0..]xs
mapWithIndex f    === (f<$>).zip[0..]           {- points free version -}

(This also gives us 1-based indexing for free!)

Often this combines well within list comprehentions, where even a builtin mapWithIndex won't help:

    [ ... | ..., (i,x)<-zip[0..]xs, ...]

Other times, you really want to use the nonexistant equivalent maximumOn of sortOn, but the import is too many bytes, or using maximumBy is too many bytes too. instead, use*:

sortOn f xs === snd$sort$(f>>=(,))<$>xs
            === snd$sort[(f x,x)|x<-xs]   {- inlinable version -}
sortOn f    === snd.sort.(f>>=(,)<$>)     {- points free version -}

Note that sometimes you will need both the best x and its f x, in which case you can get rid of three bytes and have it computed for you for free!

many other uses for this combination are possible too.