nixpkgs/pkgs/lib/debug.nix

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let lib = import ./default.nix; in
rec {
# Wrapper aroung the primop `addErrorContext', which shouldn't used
# directly. It evaluates and returns `val', but if an evaluation
# error occurs, the text in `msg' is added to the error context
# (stack trace) printed by Nix.
addErrorContext =
if builtins ? addErrorContext
then builtins.addErrorContext
else msg: val: val;
addErrorContextToAttrs = lib.mapAttrs (a : v : lib.addErrorContext "while evaluating ${a}" v);
traceVal = if builtins ? trace then x: (builtins.trace x x) else x: x;
traceXMLVal = if builtins ? trace then x: (builtins.trace (builtins.toXML x) x) else x: x;
# this can help debug your code as well - designed to not produce thousands of lines
traceShowVal = x : __trace (showVal x) x;
traceShowValMarked = str: x: __trace (str + showVal x) x;
attrNamesToStr = a : lib.concatStringsSep "; " (map (x : "${x}=") (__attrNames a));
showVal = x :
if __isAttrs x then
if x ? outPath then "x is a derivation, name ${if x ? name then x.name else "<no name>"}, { ${attrNamesToStr x} }"
else "x is attr set { ${attrNamesToStr x} }"
else if __isFunction x then "x is a function"
else if x == [] then "x is an empty list"
else if __isList x then "x is a list, first item is : ${showVal (__head x)}"
else if x == true then "x is boolean true"
else if x == false then "x is boolean false"
else if x == null then "x is null"
else "x is probably a string starting, starting characters: ${__substring 0 50 x}..";
# trace the arguments passed to function and its result
traceCall = n : f : a : let t = n2 : x : traceShowValMarked "${n} ${n2}:" x; in t "result" (f (t "arg 1" a));
traceCall2 = n : f : a : b : let t = n2 : x : traceShowValMarked "${n} ${n2}:" x; in t "result" (f (t "arg 1" a) (t "arg 2" b));
traceCall3 = n : f : a : b : c : let t = n2 : x : traceShowValMarked "${n} ${n2}:" x; in t "result" (f (t "arg 1" a) (t "arg 2" b) (t "arg 3" c));
}