fixup! Add Fuzz.filterMap

elm-explorations · Jan 12, 2024 · 55c4cd2 · 55c4cd2
1 parent fc66bf7
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Showing 2 changed files with 19 additions and 3 deletions.
diff --git a/src/Fuzz.elm b/src/Fuzz.elm
@@ -63,7 +63,7 @@ can usually find the simplest input that reproduces a bug.
 
 ## Working with Fuzzers
 
-@docs constant, invalid, filter
+@docs constant, invalid, filter, filterMap
 @docs map, map2, map3, map4, map5, map6, map7, map8, andMap
 @docs andThen, lazy, sequence, traverse
 
@@ -1341,6 +1341,22 @@ filter predicate = filterMap (\a -> if predicate a then Just a else Nothing)
 {-| A fuzzer that applies a function returning a Maybe on a given fuzzer and
 output values, as List.filterMap does.
 
+Example usage:
+
+    type UnicodeNonLetter = UnicodeNonLetter Char
+
+    fromChar : Char -> Maybe UnicodeNonLetter
+    fromChar c =
+        if (c |> Unicode.isLower |> not) && (c |> Unicode.isUpper |> not) then
+            UnicodeNonLetter |> Just
+        else
+            Nothing
+
+
+    fuzz : Fuzzer UnicodeNonLetter
+    fuzz =
+        Fuzz.char |> Fuzz.filterMap fromChar
+
 Warning: By using `Fuzz.filterMap` you can get exceptionally unlucky and get 15
 rejections in a row, in which case the test will fluke out and fail!
 

diff --git a/tests/src/FuzzerTests.elm b/tests/src/FuzzerTests.elm
@@ -1145,14 +1145,14 @@ fuzzerSpecificationTests =
                     intsNotDivBy5 : Fuzzer Int
                     intsNotDivBy5 =
                         Fuzz.int
-                            |> Fuzz.filterMap (\i -> if isDivBy5 i then Just i else Nothing)
+                            |> Fuzz.filterMap (\i -> if isDivBy5 i then Nothing else Just i)
 
                 in
                 [ rejects "impossible func (always Nothing)"
                     (Fuzz.int |> Fuzz.filterMap (\_ -> Nothing))
                     "Too many values were filtered out"
                 , passes "trivial func (always Just) doesn't reject"
-                    (Fuzz.int |> Fuzz.filterMap (\_ -> True))
+                    (Fuzz.int |> Fuzz.filterMap Just)
                     (\_ -> True)
                 , canGenerateSatisfyingWith { runs = 5000 } "not divisible by 5" intsNotDivBy5 (not << isDivBy5)
                 , cannotGenerateSatisfyingWith { runs = 5000 } "divisible by 5" intsNotDivBy5 isDivBy5