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E015: Invalid String Interpolation

Severity: Error

`int` cannot be used in string interpolation; use `toString` to convert it explicitly

A value of a type that Nix cannot interpolate was used inside "${...}". Nix string interpolation only works for:

  • strings (identity)
  • paths (converted to absolute path string)
  • derivations (attrsets with outPath — converted to store path)

It does not work for int, bool, float, null, lists, or functions — these cause a runtime error like cannot coerce an integer to a string.

How to fix

Wrap the expression in toString:

# Before (E015):
"count: ${1 + 2}"

# After:
"count: ${toString (1 + 2)}"

The LSP offers a quick fix to insert toString automatically.