It is named after logician Haskell Curry. Haskell's semantics are historically based on those of the Miranda programming language, which served to focus the efforts of the initial Haskell working group. The next formal specification is planned for Haskell is used in academia   and industry. Following the release of Miranda by Research Software Ltd.
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The known deviations from the standards are described below. This behaviour is controlled by the NondecreasingIndentation extension. GHC simply gobbles up the whole expression, parsing like this:. The Haskell Report allows you to put a unary - preceding certain expressions headed by keywords, allowing constructs like - case x of GHC does not allow this.
Instead, unary - is allowed before only expressions that could potentially be applied as a function. In its default mode, GHC makes some programs slightly more defined than they should be.
For example, consider. This should call error but actually prints True. Reason: GHC eta-expands f to. This improves efficiency slightly but significantly for most programs, and is bad for only a few. In its default mode, GHC does not accept datatype contexts, as it has been decided to remove them from the next version of the language standard.
This behaviour can be controlled with the DatatypeContexts extension. See Data type contexts. The Haskell Report specifies that a group of bindings at top level, or in a let or where should be sorted into strongly-connected components, and then type-checked in dependency order Haskell Report, Section 4.
As each group is type-checked, any binders of the group that have an explicit type signature are put in the type environment with the specified polymorphic type, and all others are monomorphic until the group is generalised Haskell Report, Section 4. In GHC the dependency analysis ignores references to variables that have an explicit type signature.
As a result of this refined dependency analysis, the dependency groups are smaller, and more bindings will typecheck. For example, consider:. Now, the definition for f is typechecked, with this type for g in the type environment.
The same refined dependency analysis also allows the type signatures of mutually-recursive functions to have different contexts, something that is illegal in Haskell 98 Section 4. GHC only insists that the type signatures of a refined group have identical type signatures; in practice this means that only variables bound by the same pattern binding must have the same context. For example, this is fine:. GHC requires the use of hs-boot files to cut the recursive loops among mutually recursive modules as described in How to compile mutually recursive modules.
This more of an infelicity than a bug: the Haskell Report says Section 5. The Num class does not have Show or Eq superclasses. Show and Eq instances, and. The Bits class does not have a Num superclasses. It therefore does not have default methods for the bit , testBit and popCount methods. Num instance, and. The reason for this is efficiency, pure and simple. Tuples are currently limited to size Specifically, the Report specifies that.
The Haskell definition of Show stipulates that the rendered string should only include parentheses which are necessary to unambiguously parse the result. For historical reasons, Show instances derived by GHC include parentheses around records despite the fact that record syntax binds more tightly than function application; e. So, for example,. A possible reason for this is that readLitChar accepts hex and octal escapes, so it seems inconsistent not to do so for integers too. The Haskell 98 definition of isAlpha is:.
In GHC the Int type follows the size of an address on the host architecture; in other words it holds 32 bits on a bit machine, and bits on a bit machine. The fromInteger and hence also fromIntegral is a special case when converting to Int. This behaviour was chosen so that for example writing 0xffffffff :: Int preserves the bit-pattern in the resulting Int. Negative literals, such as -3 , are specified by a careful reading of the Haskell Report as meaning Prelude.
So means negate fromInteger Since fromInteger takes the lower 32 bits of the representation, fromInteger Integer , computed at type Int is Int. The negate operation then overflows, but it is unchecked, so negate Int is just In short, one can write minBound::Int as a literal with the expected meaning but that is not in general guaranteed.
The fromIntegral function also preserves bit-patterns when converting between the sized integral types Int8 , Int16 , Int32 , Int64 and the unsigned Word variants , see the modules Data. Int and Data.
Word in the library documentation. In addition to those, GHC also has the following known bugs or infelicities. These bugs are more permanent; it is unlikely that any of them will be fixed in the short term.
This means that programs that do not allocate may never context switch. This is especially true of programs using STM, which may deadlock after observing inconsistent state. See Trac for further discussion. This flag ensures that yield points are inserted at every function entrypoint at the expense of a bit of performance. GHC does not allow you to have a data type with a context that mentions type variables that are not data type parameters.
For example:. The type variables mentioned in the context of the data type declaration must be among the type parameters of the data type. We have never found another class of programs, other than this contrived one, that makes GHC diverge, and fixing the problem would impose an extra overhead on every compilation. So the bug remains un-fixed. There is more background in Secrets of the GHC inliner.
On bit x86 platforms when using the native code generator, the -fexcess-precision option is always on. This means that floating-point calculations are non-deterministic, because depending on how the program is compiled optimisation settings, for example , certain calculations might be done at bit precision instead of the intended bit or bit precision. Floating-point results may differ when optimisation is turned on.
One workaround is to use the -msse2 option see Platform-specific Flags , which generates code to use the SSE2 instruction set instead of the x87 instruction set. SSE2 code uses the correct precision for all floating-point operations, and so gives deterministic results. The state hack optimization can result in non-obvious changes in evaluation ordering which may hide exceptions, even with -fpedantic-bottoms see, e.
For instance,. Compiling this program with -O results in Hello to be printed, despite the fact that evaluate should have bottomed. Compiling with -O -fno-state-hack results in the exception one would expect. Programs compiled with -fdefer-type-errors may fail a bit more eagerly than one might expect. Will emit no output, despite the fact that the ill-typed term appears after the well-typed putStrLn "Hi there.
See Trac See Trac for one example. There is known to be maleficent interactions between weak references and laziness. Particularly, it has been observed that placing a thunk containing a reference to a weak reference inside of another weak reference may cause runtime crashes.
See Trac for details. GHCi does not respect the default declaration in the module whose scope you are in. Instead, for expressions typed at the command line, you always get the default default-type behaviour; that is, default Int,Double.
When GHCi tries to load a package affected by this bug, you get an error message of the form. The workaround is to split up the.
Glasgow Haskell Compiler stable. The Glasgow Haskell Compiler License 2. Introduction to GHC 3. Release notes for version 8. Using GHCi 6. Using runghc 7. Using GHC 8. Profiling 9. Advice on: sooner, faster, smaller, thriftier GHC Language Features Foreign function interface FFI Extending and using GHC as a Library What to do when something goes wrong Debugging compiled programs Other Haskell utility programs Running GHC on Win32 systems Known bugs and infelicities Glasgow Haskell: language non-compliance
The Haskell 98 Language and Libraries Revised Report & addenda
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Haskell 98 - A non-strict, purely functional language