print type and value in "flake attr is not a derivation" errors

This turns errors like:

error: flake output attribute 'hydraJobs' is not a derivation or path

into errors like:

error: expected flake output attribute 'hydraJobs' to be a derivation or
path but found a set: { binaryTarball = «thunk»; build = «thunk»; etc> }

This change affects all InstallableFlake commands.

Change-Id: I899757af418b6f98201006ec6ee13a448c07077c

1 files changed, 7 insertions, 2 deletions

diff --git a/src/libcmd/installable-flake.cc b/src/libcmd/installable-flake.cc
index 615f70945..46bdd411b 100644
--- a/src/libcmd/installable-flake.cc
+++ b/src/libcmd/installable-flake.cc
@@ -105,9 +105,14 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
             fmt("while evaluating the flake output attribute '%s'", attrPath)))
         {
             return { *derivedPathWithInfo };
+        } else {
+            throw Error(
+                "expected flake output attribute '%s' to be a derivation or path but found %s: %s",
+                attrPath,
+                showType(v),
+                ValuePrinter(*this->state, v, errorPrintOptions)
+            );
         }
-        else
-            throw Error("flake output attribute '%s' is not a derivation or path", attrPath);
     }
 
     auto drvPath = attr->forceDerivation();