Well this is quite complicated. Lets start with the ambiguous error: <interactive>:1:1: Ambiguous type variable `b0′ in the constraint: (Arity b0) arising from a use of `arity’ Probable fix: add a type signature that fixes these type variable(s) In the expression: arity foldr In an equation for `it’: it = arity foldr Normally, without overlapping … Read more
Simon Marlow mentions another approach in GHC bug 2209 (also linked to from Bryan O’Sullivan’s answer) You can achieve the desired effect using castSTUArray, incidentally (this is the way we do it in GHC). I’ve used this option in some of my libraries in order to avoid the unsafePerformIO required for the FFI marshalling method. … Read more
ghci’s tab-completion knows. Type :set -X at a ghci prompt, then hit Tab, and you’ll get a full list. Or :set -XFoo then Tab will get you a list of all extensions beginning with Foo.
Hopefully, you have been wise enough to not break any in your global package database. Breakage there can easily mean a reinstallation of GHC is necessary. So, let us assume that the breakage is restricted to the user package db (except possibly a package or two in the global shadowed by user packages). If only … Read more
If you need to execute a script after a git pull on the client side (your machine), then a there is no reliable solution. The post-merge client-side hook (mentioned here) won’t be triggered by every pull (for instance, not in a pull rebase, not in case of fast-forward merge) So an alias wrapping both commands … Read more
To compile and execute a programming language on stock hardware you need a number of things: a compiler to translate your source language into assembly code executable by the native host a support library (aka runtime) for primitive language services, such as memory management, IO and thread management. Things that must be leveraged from lower-level … Read more
The GHC documentation has a section relating to the C pre-processor that documents some of the predefined pre-processor macros. The Cabal documentation has a section relating to conditional compilation that gives an example relating to base. If you are writing a portable package, you should be using Cabal, anyway.
Generally speaking, garbage collection is a space/time tradeoff. Give the GC more space, and it will take less time. There are (many) other factors in play, cache in particular, but the space/time tradeoff is the most important one. The tradeoff works like this: the program allocates memory until it reaches some limit (decided by the … Read more
The way GHC’s memory management is structured, in order to keep memory usage under control, a ‘major GC’ is periodically needed, during the running of the program. This is a relatively expensive operation, and it ‘stops the world’ – the program makes no progress whilst this is occurring. Obviously, it is undesirable for this to … Read more
As several of the comments have said, this is the Haskell RTS detecting an infinite loop at run-time. It cannot always detect such loops, but in simple cases it can. For example, x = x + 1 will compile just fine, but provoke an exception at run-time. (Incidentally, this is an exception – in particular, … Read more