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.. _defer-type-errors:
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"
Even though ``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 ``a``.
For more motivation and details please refer to the
:ghc-wiki:`Wiki <defer-errors-to-runtime>` page or the `original
paper <http://dreixel.net/research/pdf/epdtecp.pdf>`__.
Enabling deferring of type errors
---------------------------------
The flag :ghc-flag:`-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 :ghc-flag:`-Wno-deferred-type-errors`
to suppress these warnings.
This flag implies the :ghc-flag:`-fdefer-typed-holes` and
:ghc-flag:`-fdefer-out-of-scope-variables` flags, which enables this behaviour
for `typed holes <#typed-holes>`__ and variables. Should you so wish, it is
possible to enable :ghc-flag:`-fdefer-type-errors` without enabling
:ghc-flag:`-fdefer-typed-holes` or :ghc-flag:`-fdefer-out-of-scope-variables`,
by explicitly specifying :ghc-flag:`-fno-defer-typed-holes
<-fdefer-typed-holes>` or :ghc-flag:`-fno-defer-out-of-scope-variables
<-fdefer-out-of-scope-variables>` on the command-line after the
:ghc-flag:`-fdefer-type-errors` flag.
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
evaluating ``z`` will result in a runtime ``TypeError``.
Deferred type errors in GHCi
----------------------------
The flag :ghc-flag:`-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:
.. code-block:: none
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
Limitations of deferred type errors
-----------------------------------
The errors that can be deferred are:
- Out of scope term variables
- Equality constraints; e.g. ``ord True`` gives 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
<https://gitlab.haskell.org/ghc/ghc/issues/14605>`_).
- Kind errors in a visible type application. e.g. ::
reverse @Maybe xs
- Kind errors in a ``default`` declaration. e.g. ::
default( Double, Int Int )
- Errors involving linear types (c.f. :ghc-ticket:`20083`). e.g. ::
f :: a %1 -> a
f _ = ()
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