base: Fix handling of showEFloat (Just 0)

Previously `showEFloat (Just 0) pi ""` would produce `3.0e0`. Of
course, this
blatantly disrespects the user's request to print with zero digits of
precision.
Fix this.

This is tested by base's `num008` testcase.

Test Plan: Validate

Reviewers: hvr

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15115

Differential Revision: https://phabricator.haskell.org/D4665

3 files changed, 34 insertions, 21 deletions

diff --git a/libraries/base/GHC/Float.hs b/libraries/base/GHC/Float.hs
index d60c660bd0..db4200d874 100644
--- a/libraries/base/GHC/Float.hs
+++ b/libraries/base/GHC/Float.hs
@@ -690,6 +690,16 @@ formatRealFloatAlt fmt decs alt x
           [d]     -> d : ".0e" ++ show_e'
           (d:ds') -> d : '.' : ds' ++ "e" ++ show_e'
           []      -> errorWithoutStackTrace "formatRealFloat/doFmt/FFExponent: []"
+       Just 0 ->
+        -- handle this case specifically since we need to omit the
+        -- decimal point as well (#15115)
+        case is of
+          [0] -> "0e0"
+          _ ->
+           let
+             (ei,is') = roundTo base 1 is
+             d:_ = map intToDigit (if ei > 0 then init is' else is')
+           in d : 'e' : show (e-1+ei)
        Just dec ->
         let dec' = max dec 1 in
         case is of
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index d4b9efa2ea..f239073b60 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -17,6 +17,9 @@
   * `asinh` for `Float` and `Double` is now numerical stable in the face of
     large negative arguments (#14927)
 
+  * `Numeric.showEFloat (Just 0)` now respects the user's requested precision.
+    (#15115)
+
 ## 4.11.1.0 *TBA*
   * Bundled with GHC 8.4.2
 
diff --git a/libraries/base/tests/Numeric/num008.stdout b/libraries/base/tests/Numeric/num008.stdout
index 5086442f0f..116998cc80 100644
--- a/libraries/base/tests/Numeric/num008.stdout
+++ b/libraries/base/tests/Numeric/num008.stdout
@@ -6,14 +6,14 @@
 4.2000000e-2
 1.8217369e0
 1.8217369e-300
-0.0e0
-4.2e2
-4.2e1
-4.2e0
-4.2e-1
-4.2e-2
-1.8e0
-1.8e-300
+0e0
+4e2
+4e1
+4e0
+4e-1
+4e-2
+2e0
+2e-300
 0.0e0
 4.2e2
 4.2e1
@@ -59,9 +59,9 @@
 42
 4
 0
-4.2e-2
+4e-2
 2
-1.8e-300
+2e-300
 0.0
 420.0
 42.0
@@ -79,14 +79,14 @@
 4.2000000e-2
 1.8217369e0
 0.0000000e0
-0.0e0
-4.2e2
-4.2e1
-4.2e0
-4.2e-1
-4.2e-2
-1.8e0
-0.0e0
+0e0
+4e2
+4e1
+4e0
+4e-1
+4e-2
+2e0
+0e0
 0.0e0
 4.2e2
 4.2e1
@@ -132,7 +132,7 @@
 42
 4
 0
-4.2e-2
+4e-2
 2
 0
 0.0
@@ -144,5 +144,5 @@
 1.8217369
 0.0
 
-[0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,1.8217369e-300,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8,1.8e-300,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369128763983,1.821736912876398e-300,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.0,0.0,0.0,2.0,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369128763983,1.821736912876398e-300,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,1.8217369e-300,0.0,420.0,42.0,4.0,0.0,4.2e-2,2.0,1.8e-300,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369128763983,1.821736912876398e-300]
-[0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.0,0.0,0.0,2.0,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.0,0.0,4.2e-2,2.0,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0]
+[0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,1.8217369e-300,0.0,400.0,40.0,4.0,0.4,4.0e-2,2.0,2.0e-300,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369128763983,1.821736912876398e-300,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.0,0.0,0.0,2.0,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369128763983,1.821736912876398e-300,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,1.8217369e-300,0.0,420.0,42.0,4.0,0.0,4.0e-2,2.0,2.0e-300,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369128763983,1.821736912876398e-300]
+[0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,400.0,40.0,4.0,0.4,4.0e-2,2.0,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.0,0.0,0.0,2.0,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0,0.0,420.0,42.0,4.0,0.0,4.0e-2,2.0,0.0,0.0,420.0,42.0,4.2,0.42,4.2e-2,1.8217369,0.0]