Difference between revisions of "Avoiding partial functions"

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(Add reference to the safe library.)
 
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If you use them, you always risk to end up with an undefined.
 
If you use them, you always risk to end up with an undefined.
 
In this article we give some hints how to avoid them, leading to code that you can be more confident about.
 
In this article we give some hints how to avoid them, leading to code that you can be more confident about.
  +
  +
In addition to the below rewrites, many partial functions can be avoided by using variants from the [https://hackage.haskell.org/package/safe safe] library.
   
 
For a partial function f the general pattern is:
 
For a partial function f the general pattern is:
Whereever we write "check whether x is in the domain of f before computing f x",
+
Wherever we write "check whether x is in the domain of f before computing f x",
 
we replace it by combination of check and computation of f.
 
we replace it by combination of check and computation of f.
  +
  +
== fromJust ==
  +
  +
You should replace
  +
<haskell>
  +
if isNothing mx
  +
then g
  +
else h (fromJust mx)
  +
</haskell>
  +
by
  +
<haskell>
 
case mx of
  +
Nothing -> g
  +
Just x -> h x
  +
</haskell>
  +
which is equivalent to
  +
<haskell>
  +
maybe g h mx
  +
</haskell>
   
 
== head, tail ==
 
== head, tail ==
Line 39: Line 60:
 
viewRTotal x xs =
 
viewRTotal x xs =
 
forcePair $
 
forcePair $
case xs of
 
  +
foldr
[] -> ([], x)
+
(\x0 go y -> case go y of ~(zs,z) -> (x0:zs,z))
y:ys -> case viewRTotal y ys of (zs,z) -> (x:zs,z)
+
(\y -> ([],y))
  +
xs x
   
 
forcePair :: (a,b) -> (a,b)
 
forcePair :: (a,b) -> (a,b)
 
forcePair ~(a,b) = (a,b)
 
forcePair ~(a,b) = (a,b)
 
</haskell>
 
</haskell>
  +
  +
Alternatively, you may import from {{HackagePackage|id=utility-ht}}:
  +
* [http://hackage.haskell.org/packages/archive/utility-ht/0.0.8/doc/html/Data-Tuple-HT.html forcePair]
  +
* [http://hackage.haskell.org/packages/archive/utility-ht/0.0.8/doc/html/Data-List-HT.html viewR]
  +
* [http://hackage.haskell.org/packages/archive/utility-ht/0.0.8/doc/html/Data-List-HT.html switchR].
   
 
== (!!) ==
 
== (!!) ==
Line 80: Line 106:
 
(_,[]) -> y
 
(_,[]) -> y
 
</haskell>
 
</haskell>
  +
  +
  +
== minimum ==
  +
  +
The function <hask>isLowerLimit</hask> checks if a number is a lower limit to a sequence.
  +
You may implement it with the partial function <hask>minimum</hask>.
  +
<haskell>
  +
isLowerLimit :: Ord a => a -> [a] -> Bool
  +
isLowerLimit x ys = x <= minimum ys
  +
</haskell>
  +
It fails if <hask>ys</hask> is empty or infinite.
  +
  +
You should replace it by
  +
<haskell>
  +
isLowerLimit x = all (x<=)
  +
</haskell>
  +
This definition terminates for infinite lists, if <hask>x</hask> is not a lower limit. It aborts immediately if an element is found which is below <hask>x</hask>.
  +
Thus it is also faster for finite lists.
  +
Even more: It also works for empty lists.
  +
  +
  +
[[Category:Tutorials]]
  +
[[Category:Style]]
  +
[[Category:Idioms]]

Latest revision as of 04:03, 4 September 2017

There are several partial functions in the Haskell standard library. If you use them, you always risk to end up with an undefined. In this article we give some hints how to avoid them, leading to code that you can be more confident about.

In addition to the below rewrites, many partial functions can be avoided by using variants from the safe library.

For a partial function f the general pattern is: Wherever we write "check whether x is in the domain of f before computing f x", we replace it by combination of check and computation of f.

fromJust

You should replace

if isNothing mx
  then g
  else h (fromJust mx)

by

case mx of
   Nothing -> g
   Just x -> h x

which is equivalent to

maybe g h mx

head, tail

You should replace

if null xs
  then g
  else h (head xs) (tail xs)

by

case xs of
   [] -> g
   y:ys -> h y ys

init, last

You may replace

if null xs
  then g
  else h (init xs) (last xs)

by

case xs of
   [] -> g
   y:ys -> uncurry h $ viewRTotal y ys

viewRTotal :: a -> [a] -> ([a], a)
viewRTotal x xs =
   forcePair $
   foldr
      (\x0 go y -> case go y of ~(zs,z) -> (x0:zs,z))
      (\y -> ([],y))
      xs x

forcePair :: (a,b) -> (a,b)
forcePair ~(a,b) = (a,b)

Alternatively, you may import from utility-ht:

(!!)

You should replace

if k < length xs
  then xs!!k
  else y

by

case drop k xs of
   x:_ -> x
   [] -> y

This is also more lazy, since for computation of length you have to visit every element of the list.


irrefutable pattern match on (:)

You should replace

if k < length xs
  then let (prefix,x:suffix) = splitAt k xs
       in  g prefix x suffix
  else y

by

case splitAt k xs of
   (prefix,x:suffix) -> g prefix x suffix
   (_,[]) -> y


minimum

The function isLowerLimit checks if a number is a lower limit to a sequence. You may implement it with the partial function minimum.

isLowerLimit :: Ord a => a -> [a] -> Bool
isLowerLimit x ys = x <= minimum ys

It fails if ys is empty or infinite.

You should replace it by

isLowerLimit x = all (x<=)

This definition terminates for infinite lists, if x is not a lower limit. It aborts immediately if an element is found which is below x. Thus it is also faster for finite lists. Even more: It also works for empty lists.