Difference between revisions of "Avoiding partial functions"
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CaleGibbard (talk  contribs) m 

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For a partial function f the general pattern is: 
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. 
we replace it by combination of check and computation of f. 

Revision as of 16:17, 22 March 2016
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.
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 utilityht:
(!!)
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.