# Difference between revisions of "Avoiding partial functions"

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(fromJust) |
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Whereever we write "check whether x is in the domain of f before computing f x", |
Whereever we write "check whether x is in the domain of f before computing f x", |
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we replace it by combination of check and computation of f. |
we replace it by combination of check and computation of f. |
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+ | |||

+ | == fromJust == |
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+ | |||

+ | You should replace |
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+ | <haskell> |
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+ | if isJust mx |
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+ | then g |
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+ | else h (fromJust mx) |
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+ | </haskell> |
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+ | by |
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+ | <haskell> |
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+ | case mx of |
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+ | Nothing -> g |
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+ | Just x -> h x |
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+ | </haskell> |
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== head, tail == |
== head, tail == |

## Revision as of 12:10, 5 June 2012

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: Whereever 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 isJust mx
then g
else h (fromJust mx)
```

by

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

## 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)
```

## (!!)

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
```