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| -rw-r--r-- | README.md | 2 | ||||
| -rw-r--r-- | ch04/4_b_3.hs | 58 |
2 files changed, 59 insertions, 1 deletions
@@ -74,7 +74,7 @@ more visible in the list the first exercise of a group is in bold italics. | 4_a_4 | yes | | | | **_4_b_1_** | yes, in 4_b_2 | 97 | | | 4_b_2 | yes | 98 | | -| 4_b_3 | | | | +| 4_b_3 | yes | | | | 4_b_4 | | | | | 4_b_5 | | | | | 4_b_6 | | | | diff --git a/ch04/4_b_3.hs b/ch04/4_b_3.hs new file mode 100644 index 0000000..9185f4e --- /dev/null +++ b/ch04/4_b_3.hs @@ -0,0 +1,58 @@ +-- Extend your function to handle the following kinds of exceptional conditions +-- by calling error. +-- +-- ghci> asInt_fold "" +-- 0 +-- ghci> asInt_fold "-" +-- 0 +-- ghci> asInt_fold "-3" +-- -3 +-- ghci> asInt_fold "2.7" +-- *** Exception: Char.digitToInt: not a digit '.' +-- ghci> asInt_fold "314159265358979323846" +-- 564616105916946374 + +-- The assignment is ambiguous. It tells us we should call error, but the +-- examples show only one example of an error situation (for input "2.7"). This +-- error is already caught and reported by the digitToInt function, producing +-- the same error as shown in the examples, so no work from us is needed to make +-- the function to behave as required. +-- +-- However, the next exercise replaces the call to error by returning Either. +-- +-- Let's make the function to behave as specified in assignment of this +-- exercise, calling error producing the same error output as shown in the next +-- exercise, thus preparing the code for that exercise. +-- +-- ghci> asInt_fold "2.7" +-- *** Exception: non-digit '.' + +import Data.Char (digitToInt, isDigit) + +asInt_fold :: String -> Int +asInt_fold "" = 0 +asInt_fold "-" = 0 +asInt_fold ('-':xs) = -1 * (asInt_fold' xs) +asInt_fold xs = asInt_fold' xs + +asInt_fold' :: String -> Int +asInt_fold' xs = foldl nextDigit 0 xs + where nextDigit acc digit + | not (isDigit digit) = error ("non-digit '" ++ [digit] ++ "'") + | otherwise = (acc * 10) + (digitToInt digit) + +-- ghci> :l 4_b_3.hs +-- [1 of 1] Compiling Main ( 4_b_3.hs, interpreted ) +-- Ok, one module loaded. +-- ghci> asInt_fold "" +-- 0 +-- ghci> asInt_fold "-" +-- 0 +-- ghci> asInt_fold "-3" +-- -3 +-- ghci> asInt_fold "2.7" +-- *** Exception: non-digit '.' +-- CallStack (from HasCallStack): +-- error, called at 4_b_3.hs:41:35 in main:Main +-- ghci> asInt_fold "314159265358979323846" +-- 564616105916946374 |