-fdefer-type-errors

Foo.hs

(a,b) = ("foo", "bar" + 42)

ghci-session

% ghci -fdefer-type-errors Foo.hs
[1 of 1] Compiling Main             ( Foo.hs, interpreted )

Foo.hs:1:1: Warning:
    Top-level binding with no type signature: a :: [Char]

Foo.hs:1:1: Warning:
    Top-level binding with no type signature: b :: [Char]

Foo.hs:1:23: Warning:
    No instance for (Num [Char]) arising from a use of ‘+’
    In the expression: "bar" + 42
    In the expression: ("foo", "bar" + 42)
    In a pattern binding: (a, b) = ("foo", "bar" + 42)
Ok, modules loaded: Main.

λ> a
"foo"

λ> b
"*** Exception: Foo.hs:1:23:
    No instance for (Num [Char]) arising from a use of ‘+’
    In the expression: "bar" + 42
    In the expression: ("foo", "bar" + 42)
    In a pattern binding: (a, b) = ("foo", "bar" + 42)
(deferred type error)