6.4.19. Deferring type errors to runtime¶
While developing, sometimes it is desirable to allow compilation to succeed even if there are type errors in the code. Consider the following case:
module Main where a :: Int a = 'a' main = print "b"
a is ill-typed, it is not used in the end, so if all
that we’re interested in is
main it can be useful to be able to
ignore the problems in
For more motivation and details please refer to the Wiki page or the original paper.
220.127.116.11. Enabling deferring of type errors¶
-fdefer-type-errors controls whether type errors are
deferred to runtime. Type errors will still be emitted as warnings, but
will not prevent compilation. You can use
to suppress these warnings.
This flag implies the
-fdefer-out-of-scope-variables flags, which enables this behaviour
for typed holes and variables. Should you so wish, it is
possible to enable
-fdefer-type-errors without enabling
by explicitly specifying
-fno-defer-out-of-scope-variables on the command-line after the
At runtime, whenever a term containing a type error would need to be
evaluated, the error is converted into a runtime exception of type
TypeError. Note that type errors are deferred as much as possible
during runtime, but invalid coercions are never performed, even when
they would ultimately result in a value of the correct type. For
example, given the following code:
x :: Int x = 0 y :: Char y = x z :: Int z = y
z will result in a runtime
18.104.22.168. Deferred type errors in GHCi¶
-fdefer-type-errors works in GHCi as well, with one
exception: for “naked” expressions typed at the prompt, type errors
don’t get delayed, so for example:
Prelude> fst (True, 1 == 'a') <interactive>:2:12: No instance for (Num Char) arising from the literal `1' Possible fix: add an instance declaration for (Num Char) In the first argument of `(==)', namely `1' In the expression: 1 == 'a' In the first argument of `fst', namely `(True, 1 == 'a')'
Otherwise, in the common case of a simple type error such as typing
reverse True at the prompt, you would get a warning and then an
immediately-following type error when the expression is evaluated.
This exception doesn’t apply to statements, as the following example demonstrates:
Prelude> let x = (True, 1 == 'a') <interactive>:3:16: Warning: No instance for (Num Char) arising from the literal `1' Possible fix: add an instance declaration for (Num Char) In the first argument of `(==)', namely `1' In the expression: 1 == 'a' In the expression: (True, 1 == 'a') Prelude> fst x True
22.214.171.124. Limitations of deferred type errors¶
The errors that can be deferred are:
- Out of scope term variables
- Equality constraints; e.g.
ord Truegives rise to an insoluble equality constraint
Char ~ Bool, which can be deferred.
- Type-class and implicit-parameter constraints
All other type errors are reported immediately, and cannot be deferred; for example, an ill-kinded type signature, an instance declaration that is non-terminating or ill-formed, a type-family instance that does not obey the declared injectivity constraints, etc etc.
In a few cases, equality constraints cannot be deferred. Specifically:
Kind errors in a type or kind signature, partial type signatures, or pattern signature. e.g.
f :: Int Bool -> Char
This type signature contains a kind error which cannot be deferred.
Type equalities under a forall (c.f. #14605).
Kind errors in a visible type application. e.g.
reverse @Maybe xs
Kind errors in a
default( Double, Int Int )