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Haskell programming hints

Shadowing prelude functions

Sometimes you may want to define a variable or parameter with a nice name like init or tail, only for GHC to yell at you that you are shadowing some function you've never heard of:

This binding for ‘init’ shadows the existing binding
      imported from ‘Prelude’ at ...
      (and originally defined in ‘GHC.List’)

This is because functions with these names are exposed from the Prelude module, which is implicitly imported into every Haskell module. We can hide names from Prelude by explicitly importing it, like any other module, and specify the names we don't want to get:

import Prelude hiding (init)

Warnings in ghci

By default, ghci (the Haskell REPL) will not print warnings about your code. This is unfortunate, since many of the warnings are useful. We can make ghci print warnings by using the :set command at the ghci prompt:

λ> :set -Wall

Unfortunately, some warnings are quite noisy for interactive use. In particular the one that warns about type defaults:

λ> 2+2

<interactive>:191:1: warning: [GHC-18042] [-Wtype-defaults]
    • Defaulting the type variable ‘a0’ to type ‘Integer’ in the following constraints
        (Show a0) arising from a use of ‘print’ at <interactive>:191:1-3
        (Num a0) arising from a use of ‘it’ at <interactive>:191:1-3
    • In a stmt of an interactive GHCi command: print it
4

This warning arises because the literal 2 is polymorphic (can have any integral type), and barring any external constraints, ghci defaults to assigning it the type Integer. This can be a useful warning when writing application code, but is a bit annoying when playing around in ghci. We can silence this specific warning as follows:

λ> :set -Wno-type-defaults

Accessing modules in cabal repl

Plain ghci works only when you use exclusively the standard library. In other cases, we must use cabal repl to also load any dependencies specified in the .cabal file. After running cabal repl, your modules have been loaded, but their definitions are not available at the REPL. A module Foo.Bar can be made available via the :m command, as follows:

> :m *Foo.Bar

The asterisk makes unexported definitions available as well, which is usually desirable when testing.

After modifying the module, use :r (without arguments) to reload any changes.

Useful modules

AP is not a course that requires much knowledge about the Haskell base library, but there are some modules that contain utility functions you may find useful in your work: